The pair clearance is inevitably exists in the mechanical system. With the speed of mechanical system increasing, clearance does harm to the stability and precision of mechanical system. Using four-bar linkage as the example to research the clearances’ characters, a dynamics model is established based on the kinematics and dynamics relations of the mechanism. Through the dynamics model, the track of pin center to collar in pair can be gotten. The irregularity of pin center track is related to the clearance and velocity and seriously influences the mechanical performance. So, it has to pay more attention to the clearance, especially in high speed.