In the past, the model of undersea piston engine system can only be obtained by experiment. And based on the power process of piston machine studied by numerical simulation method and in consideration of the influence of overexpansion on the output performance of piston engine, a fitting formula of indicated work is established. Through the output torque of the piston machine derived by the indicated work fitting formula to subtract the torque consumed by the auxiliary machine in combination with the complete gas equation of state, a dynamic system model of the undersea piston engine is obtained. In comparison the designed simulation program by the model with the semi-physical simulation experiment, the maximum error between the simulation results of the mathematical model and the semi-physical simulation experiment results is 5.28%. The results show that the model of the undersea piston power system based on the combination of numerical study and theoretical derivation can be used for the preliminary verification of the undersea power system characteristics instead of the experiment.