A finite element model of a piston and a block surface was established. After boundary and work conditions were applied, an initial transient analysis of the model was carried out. The simulation result of vibration acceleration on the block surface conformed well with the experiment, thus validating the model. Based on the model, the motion law of pistons in blocks was studied. The variation trend of the piston slap force, piston clearance on the piston slap force, and the effect of piston pin offset on the piston slap force at different crankshaft speeds were analyzed. Results show that suitable piston pin offset and smaller piston clearance reduce the piston slap force quite well; and the contribution of the piston force at the point when gas breaks out is dominant at low speed, which is not obvious at high speed. The findings provide evidence for the performance improvement of internal combustion engine development. 