1. A harmonic-characteristics based current pattern matching method for the non-intrusive power load monitoring, wherein, establishing a load signature database which comprising the following load characteristic information: (1) the power state and operating state of different appliances under the fundamental wave reference voltage U ref ; (2) the steady-state current harmonic parameters of each operating state of different appliances; and comprising the following steps: Step of Electrical Appliance Registration and Initialization of Load State Word Space: 1) Determining the appliances inside the total load, and acquiring the load characteristic information of each appliance from the load signature database, and achieving the electrical appliance registration; 2) Statistical analyzing all the operating states of the total load, and storing the results in the orderly linear table of [state] in the form of state word to finish the initialization of load state word space Ω SW ; Step of data acquisition and data preprocessing: This step includes sampling the load terminal voltage and steady-state total current, and denoising and harmonic analysis of the measured voltage and total current signal, thereby the following results are obtained: the measured total fundamental active power P l 1 (t) is of: 
 P l 1 ( t )= U 1 I 1 cos(θ l1 )  (6) the measured total fundamental reactive power Q l 1 (t) is of: 
 Q l 1 ( t )= U 1 I 1 sin(θ l1 )  (7) the measured unit total current pattern I l (P l 1 (t),Q l 1 (t)) is of: 
 I l ( P l 1 ( t ), Q l 1 ( t ))=(1·∠θ l1 ,. .. ,α lh ·∠θ lh ,. .. ,α lH ·∠ lH ) T (15) in equation (6) and (7), U 1 represents the fundamental wave RMS-value of the measured load terminal voltage u(t), I 1 represents the fundamental wave RMS-value of the measured load total current i l (t); θ l1 represents the initial phase angle of the fundamental wave of current i l (t) relative to the fundamental phase angle of load terminal voltage; in equation (15), α lh represents per unit value of the h-th harmonic amplitude of current i l (t) with its fundamental amplitude as base value, so α l 1 =1; θ lh represents the initial phase angle of the h-th harmonic of current i l (t) relative to the fundamental phase angle of load terminal voltage; Step of feasible state word space search based on table looking-up: By searching the orderly linear table [state], initial selection of the total load operating state are carried out in the load state word space Ω SW based on the measured fundamental active total power P l 1 (t) and the measured fundamental reactive total power Q l 1 (t), and consequently the feasible state word space Ω SW (P l 1 (t),Q l 1 (t))⊂Z N meeting the fundamental total power constraints is obtained; wherein, Z represents integer number space; Step of the optimal matching of current pattern: The objective function of the optimal matching of current pattern is as follows: 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 ( 16 ) in equation (16), ∥•∥ represents the L 2 norm; In the feasible state word space Ω SW (P l 1 (t),Q l 1 (t))⊂Z N , a state word vector SW min (t) bringing the objective function of the optimal matching of current pattern to obtain the minimum value is searched, which serves as the optimal estimation of the current operating state of the total load, then the accordingly appliance operating state is identified, and the P min 1 (t) which exclusively corresponds to the SW min (t) serves as the optimal estimation of the current power state of the total load, so far the disaggregation of total fundamental active power is completed; Step of display and output of the monitoring and disaggregation results: Finally, displaying and outputting the power state and operating state of each appliance inside the load are performed.