For the requirement of high-power transmit signals in long-distance submarine detection, a high-power siticon controlled rectifier switching power amplifier (SPA) was used. The operating principle of this SPA was analyzed and the equivalent circuit models were established under different working states including critical commutating state, natural commutating state and compulsive commutating state. The equivalent circuit models were emulated in OrCAD when the SPA worked in critical commutating state. For the acquisition of power amplifier and load current value about SPA in natural and compulsive commutating state, the nonlinear mathematical models were established in the base of equivalent circuit models, then they were solved by fourth-order Runge-Kutta method under the platform of Matlab and VC++. The methods mentioned can calculate the voltage and current at any time any point of the SPA; it can reflect the working state of circuit objectively and factually; the research may be of certain reference for engineering application.