Seabed drilling rig is indispensable technical equipment for seabed resource exploration， marine geological survey and marine scientific investigation， and the rotation system is one of the core components of seabed drilling rig for providing rotational speed and torque for the drill bit. In the hydraulic system with quantitative pump oil supply， a differential relief valve is placed in the rotation system to reduce the influence of load fluctuation on the output speed of the system. The rotation hydraulic system of subsea drilling rig is modeled and simulated by AMESim at step load. The simulation results show that the constant differential relief valve can always make the pump outlet pressure match the load， effectively improve the stability of output speed， and at the same time realize system energy saving.