A mathematical model of the double screw vice of spiral gear rotary hydraulic cylinder is established, and the dynamic analysis is carried out by ADAMS simulation software. Its maximum angle and output torque under the limit external load were studied. Based on the dynamic model, the key parameters of helical pair were optimized to analyze the influence of different spiral angle on the output performance of the double transmission. The results show that the transmission efficiency is the highest when the spiral angle is about 50 °. The test rig of spiral gear rotary hydraulic cylinder was set up, and the accuracy of the dynamic model is verified through the prototype test.