1. A method of three-dimensional optimization design for an asymmetric cusp magnetic field in an MCZ single crystal furnace, comprising an optimization design for both structural parameters of the magnetic field and specification parameters of coils, wherein the optimization design for structural parameters of the magnetic field is implemented according to the following steps: establishing a three-dimensional magnetic model (1) by using an Ansys numerical analysis software in step 1; and varying parameters of the three-dimensional magnetic model (1), analyzing the relationship of a magnetic induction intensity of the magnetic field varying with respect to a span between the upper and lower parts of coils, the number of transverse turns of coils and a thickness of a shield, determining the span between the upper and lower parts of coils, the number of transverse turns of coils, and the thickness of the shield based on a set magnetic induction intensity of the magnetic field, and under a condition that the magnetic field has a certain power, determining the numbers of longitudinal layers of coils in the upper and lower parts of the magnetic field by taking a common region where the magnetic induction intensity at a solid-liquid interface has both a radial component of Br≧BrminGs and an axial component of Bz≦BzmaxGs being the widest as the optimization object in step 2, wherein the optimization design for specification parameters of the coils is implemented according to the following steps: in step i, establishing a mathematic expression of heat and cross-section area of the coils as follows: w 1 = I 2 l s ρ 0 ( 1 + at ) , in the above expression, I refers to a current of the coils powered-on, l refers to a total length of the coils, s refers to a cross-section area of the coils, ρ 0 refers to a resistivity at 0, a refers to a temperature coefficient of resistivity, and t refers to an actual temperature of copper pipe walls of the coils; Establishing a mathematic expression of heat transfer w2 of the coils at the copper pipe walls and a characteristic size de, an average flow velocity V of fluid, and a temperature difference Δt between the copper pipe walls and cooling water as follows, w 2 = 0.023 λ A de ( Vde v ) 0.8 Pr 0.4 Δ t , in the above expression, λ refers to a heat conductivity of water, de refers to a characteristic size, A refers to a total heat transferring area of the copper pipe walls, V refers to an average flow velocity of fluid, v refers to a kinematic viscosity of the fluid, Pr refers to Prandtl number, Δt refers to temperature difference between the copper pipe walls and the cooling water; and establishing a mathematical expression among the amount of heat absorbed by the cooling water w3 and the number of branches of cooling water paths n, a flow amount of the cooling water q m , a specific heat capacity of water c, an outlet water temperature t2, and an inlet water temperature t1 as follows: 
 w 3 =q m c ( t 2 −t 1)· n, in the above expression, q m refers to the flow amount of cooling water, c refers to the specific heat capacity of water, t2 refers to the water temperature at in outlet, t1 refers to the water temperature at an inlet, and n refers to the number of branches of cooling water paths, and in step ii, establishing an optimization model of a system when w1=w2=w3 as follows: objective function : min ( w 1 ) = I 2 l s ρ 0 ( 1 + at ) , with constraint conditions of Δt+t2<40, and hf<20, wherein Δt+t2 is the temperature of the copper pipe walls, and hf refers to the linear loss along the coils, and wherein the cross-section area of the coils s, the characteristic size de, the average flow velocity cross-section area of the coil of the fluid V, the temperature difference Δt between the copper pipe walls and the cooling water, the number of branches of cooling water paths n, the flow amount of the cooling water q m , the specific heat capacity of water c, the outlet water temperature t2, and the inlet water temperature t1 are obtained by optimizing the specification parameters of the coils in the magnetic field using the Ansys software.