Abstract: To better understand the piston secondary motion and tribological behaviors under different operating conditions, this paper presented a coupling model of the piston secondary dynamics and the fluid dynamic lubrication. Effects of the variation in the system inertia were considered in the model to study the transient behavior of piston secondary motion and piston skirt friction and friction loss. The variations of the engine speed, the piston secondary motion and piston skirt friction loss in the startup were simulated. The piston lateral motion and the piston skirt friction force in the startup and in the steady-state were analyzed. Results showed that the model can be applied to transient study the piston secondary motion and the piston skirt tirbological behavior.