When the piston moves in the cylinder, on the one hand influenced by the explosion pressure of the gas, on the other hand due to the large mass of the piston, the piston is subjected to a large inertial force, then the force caused by the two side force on the piston. The change of lateral force will cause a slight lateral movement of the piston and the rocking motion (secondary movement) in the cylinder. The change of instantaneous clap force has a great impact on the form of oil film, which will affect the distribution of the oil film distribution. The instantaneous clap force of the piston skirt which is measured on a test machine will be considered to modular calculation of the dynamic equations. the distribution of oil film pressure will be analyzed and compared under the circumstances of considering the instantaneous clap force and not considering the instantaneous clap force, which will lay a solid foundation for the design of profile of the piston skirt as well as improve the lubrication conditions and reduce the impact on the cylinder.