The escapement mechanism in a mechanical watch movement plays a critical role. Its purpose is to maintain and count the oscillations of the balance wheel. The timekeeping accuracy is therefore regulated. The motion of the escapement mechanism is highly complicated and no in-depth study on its kinematics has been done by now. This paper investigates the kinematics of the Swiss lever escapement which is used in almost all mechanical watches. One half cycle of the motion is described in different phases. Mathematical models for each key component are developed. Simulation result shows that the model is precise in describing and predicting the motion of the escapement. This method is essential for improving and optimizing the design.