The gear mechanism for transforming bidirectional torque into unidirectional torque is the key technology of the gear transmission round-trip piston power machine.By using CAXA（Enterprise Edition）,the simulation,processing and kinematics simulation analysis of the dynamic model are carried out.Through the addendum modification of the engaging-out gear tooth or adding the addendum modification of engaging-in gear tooth,can create a gap does not interfere with the rotation of the gear,prevent gear tooth collision and determine the piston stroke.It is fully meet the requirements of strength and contact ratio of the gear force transforming mechanism.It supports the implementation of the reciprocating piston internal combustion engine on the technically.