Abstract: To investigate the influence of operating conditions of an engine and piston-liner clearance on piston slap force, this paper builds a piston motion simulation model and an engine multi-body dynamics simulation model based on the EXCITE software.The characteristics of the piston secondary motion and the piston slap force are studied through the verified models. The models are also used to analyze the effects of operating conditions like engine speed and torque as well as piston-liner clearance. The results show that the piston secondary motion is most obvious near the top dead center of the compression stroke and the maximum value of piston slap force appears at the main thrust side. With the increase of the rotational speed, the piston inertial force increases, which makes the piston slap force increase. With the increase of the torque, the lateral force of the piston increases, which strengthens the piston slap force. The piston secondary motion intensifies with the increase of the piston-liner clearance and this contributes to the increase of the piston slap force.