The stress state and the variation law of velocity in the falling process of rock breaking hammer are studied according to the immersed tunnel foundation trench in the Shenzhen-Zhongshan passage project. The FEM method is used to simulate the process of rock drilling, study the damage development characteristics of rock and analyze the influence law of water depth, impact velocity and drilling times on the rock failure and damage development. The results show that during the falling process, the velocity of rock breaking hammer increases linearly first, then decreases gradually, and finally tends to a stable ending velocity. The speed of the hammer decreases and the damage accumulates in the process of drilling. In addition, the water depth is negatively correlated with the rock damage, while the impact velocity and impact times are positively correlated with the rock damage.