Hydraulic grinding saw is a kind of important equipment broadly used in billet precise cutting in metallurgical industry. In order to improve productivity significantly, control feeding speed reasonably, at the same time to compensate the variation of driving torque caused by the change of contacting area between grinding wheel and slab, thus to obtain the optimized match between the feed of the grinding wheel and the driving motor, the application of hydraulic servo system in the control of the feeding speed of grinding wheel is discussed in this thesis. This article introduces the principle of hydraulic grinding saw, brings out the project of constant power PID control based on the relationship between driving torque and motor current. The transfer function of the hydraulic servo system, the optimized PID controller is also worked out. According to the result of simulation analysis, the controlling factors can be well obtained.