The invention discloses a semi-active energy regenerative suspension shock absorber based on mixed excitation and a size determining method of the shock absorber. The shock absorber comprises an upper hoisting lug, a dust cover, a lower hoisting lug, a hydraulic shock absorber body and a mixed excitation mechanism. The hydraulic shock absorber body provides the constant viscous damping coefficient. The mixed excitation mechanism provides the adjustable electromagnetic damping force and converts vibration energy into electric energy for storage. Meanwhile, the invention provides the size determining method. The model selection of the hydraulic shock absorber body is determined according to the rating requirements, an electromagnetic damping force expression is derived, the primary size of the mixed excitation mechanism is determined through a particle swarm optimization algorithm, finite element analysis is conducted, the gas length is optimized, and the final size is determined. According to the shock absorber and the size determining method, the hydraulic shock absorber body and a mixed excitation cylindrical linear motor are integrated, the damping force is adjustable, energy is recovered in the working process, the structure is simple, the shock isolation performance and the energy regenerative performance of a vehicle suspension are coordinated, and the Fail-safe function is also achieved; and in addition, the size determining method is clear in step, easy to implement and obvious in optimization result, and has high practicalness.