Virtual prototype technology is a new method in R&D，which can effectively improve the product quality，decrease the new product developing time and reduce the production cost. A virtual prototype of rotary actuator is built up for simulation. According to double involute-spiral-pairs，which are the key parts of the hydraulic rotary actuator，a research is done on the relationship between input pressure and output torque. After simulation and experiment，it’s found that system pressure and output torque is in a positive linear programming. And another experiment is carried out on the relationship between different helix angles and output torques under same pressure. Through three series simulations within helix angles of 30°，45° and 60°，it’s found that we’d better design a helix gear in around 45° helix angle without any gear modification，so that we can get a optimized output torque. If a bigger output torque is wanted without higher pressure，the initial way is to increase the gear radial dimension.