To control the inertial forces and moments, the new type of the reciprocating cam engine with counter-position placement has been invented. Such a thermodynamic model based on the kinematic and dynamic analysis is developed and the engine performance is simulated. Under the same structural design parameters, the counter-position placement cam engine would have the inadequate work due to the increase of throttle loss. Then effects of the gas distributions to the dynamic performance of the engine are researched. Six variables are selected to optimize maximum average indicated power and the minimum indicated specific consumption with the method of discrete variable gridding. The results indicate that: (a) The new engine structure design can present advantage of excitation forces balance. (b) The structural design parameters and the thermal efficiency of the engine should be optimized especially those of the valve actuating mechanism to achieved desired power. (c) The parameters optimized of valve train can conform to the requirements of speed and economy, and it is feasible and reasonable to put forward the scheme of cam engine with the counter-position placement.