The shaft torsional vibration of control system would cause the oscillation of system control quantity, which would increase mechanical transmission gear wears, and even led to the shaft fracture. The shaft torsional vibration could usually be suppressed by using notch filter on industrial application, but the selection of notch frequency was not considering the factor of control system delay. The influence of control system delay on shaft torsional vibration was analyzed by using the phase diagram of angular velocity increment, shaft torque increment and motor torque increment. Simulation results validated the analytical conclusions.