1. A harmonic-characteristics based current pattern matching method performed with a monitor device for the non-intrusive power load monitoring, comprising steps: (a) establishing a load signature database of household appliances, said database comprising information about (i) power states and operating states which each appliance of said various appliances has, and (ii) steady-state current harmonic parameters of each of said operating states of each said appliance; (b) registering all electrical appliances inside a total household load and initializing a linear load state word space table in an on-board memory module of said monitor device by (i) acquiring load characteristic information for each of said electrical appliances from said load signature database and (ii) performing statistical analysis on all possible operating states of said total household load and storing results of said statistical analysis in said memory module in the form of a linear state word table; (c) performing data acquisition and data preprocessing by measuring terminal voltage and steady-state total current of said household load, which are then processed in a processing module in said monitor device to obtain a measured total fundamental active power value, a total fundamental reactive power value and a unit total current pattern; (d) performing a looking-up search on said linear state word table to obtain a feasible state word space based on said measured total fundamental active power value and said total fundamental reactive power value; (e) performing an optimal matching of current pattern based on the following equation: min ∀ SW k ∈ Ω sw ⁡ ( P l 1 ⁡ ( t ) , Q l 1 ⁡ ( t ) ) ⁢  I l ⁡ ( P l 1 ⁡ ( t ) , Q l 1 ⁡ ( t ) ) - I ^ l ⁡ ( SW k , U 1 )  2 to obtain conresponding SW min (t) and P 1 min (t) values in said feasible state word space and, by disaggregating said total fundamental active power value based on said SW min (t) and P 1 min (t) values, to obtain the information of the power state and operating state for each of said electrical appliances inside said household load at the given point of time when terminal voltage and steady-state total current of said household load are measured; and (f) displaying said information of the power state and operating state on a screen of said monitor device; wherein in step (a) said steady-state current harmonic parameters in step (a) is determined based on the following equation: H ai = [ 1 ⁢ ∠θ 1 , 1 … 1 ⁢ ∠θ 1 , s ⁡ ( i ) … 1 ⁢ ∠θ 1 , Si ⋮ … ⋮ … ⋮ α h , 1 ⁢ ∠θ h , 1 … α h , s ⁡ ( i ) ⁢ ∠θ h , s ⁡ ( i ) … α h , Si ⁢ ∠θ h , Si ⋮ … ⋮ … ⋮ α H , 1 ⁢ ∠θ H , 1 … α H , s ⁡ ( i ) ⁢ ∠θ H , s ⁡ ( i ) … α H , Si ⁢ ∠θ H , Si ] where ai is appliances, Si is operating states of each appliance, H is the maximum harmonic order, h is harmonic order, α h,s(i) or per unit value of h-th harmonic amplitude of steady-state current under state s(i) of applicance ai with it fundamental amplitde as base value, θ h,s(i) is initial phase angle of h-th harmonic of steady-state current under state s(i) of appliance ai relative to fundamental phase angle of appliance terminal voltage measured; wherein in step (b) said statistical analysis is performed on total load power state, which is expressed as power vectors P 1 (t) R N and Q 1 (t) R N , consisting of fundamental powers consumed by N appliances as determined by the following two equations: 
 P 1 ( t )=( P 1 1 ,. .. P 1 i ,. .. P 1 N ) T 
 and 
 Q 1 ( t )=( Q 1 1 ,. .. Q 1 i ,. .. Q 1 N ) T , where superscript “1” is fundamental wave, N is the number of appliances inside said total load, i is individual appliance ai; said statistical analysis is performed further on N-dimensional state word vector SW(t) Z N as expressed in equation: SW(t)=(s( 1 ),. .. , (s)(i),. .. s(N)) T , which represent all possible operating states of said total household load and stored in a linear table in an order according to a reference value of total fundamental active power corresponding to each state word vector; and wherein in step(c) said measured total fundamental active power P 1 i (t) equals to U 1 I 1 cos(θ l1 ), said measured total fundamental reactive power Q 1 i (t) equals to U 1 I 1 sin( l1 ), and said unit total current pattern Il(P 1 l (t), Q 1 l (t)) equals to (1·∠θ l1 ,. .. , α lh ·∠θ lh ,. .. α lH ·∠ lH) T ,where U 1 is fundamental wave RMS-value of measured load terminal voltage u(t), I 1 is fundamental wave RMS-value of measured load total current i(t), l1 is initial phase angle of fundamental wave of current i l (t) relative to fundamental phase angle of load terminal voltage, lh is per unit value of h-th harmonic amplitude of current i l (t) with its fundamental amplitude as base value, lh is initial phase angle of h-th harmonic of current i l (t) relative to fundamental phase angle of load terminal voltage.