This paper is concerned with the internal leakage characteristics of the gripper cylinder in Tunnel Boring Machine (TBM) under impact load. The internal leakage model of the gripper cylinder included factors such as the clearance between the piston and the internal wall of the cylinder, as well as the moving amplitude and frequency of the gripper cylinder is established. Due to the leakage model, the influences of the aforementioned parameters on the leakage characteristics are analyzed. The analysis indicates that the moving amplitude and the frequency of the gripper cylinder both have remarkable influences on the internal leakage of the gripper cylinder, the internal leakage flow rate will be negative when the frequency is greater than 120 rad/s as the clearance is 0.01 mm. The peak value of the pressure in rod chamber increases while the amplitude is declining with the clearance.