Aiming at the problems of insufficient consideration of time delay in the modeling process of servo motor drive system, lack of analysis given in the premise of system stability in the calibration of control gain, and relatively high conservatism of system stability criterion, this paper proposes a time delay related stability criterion with lower conservatism. Firstly, the time-varying delay is considered in the inertia structure model and the state space equations of the time-delayed closed-loop systems are established by designing the corresponding controllers, and then the Lyapunov generalized functional analysis method is used in combination with the techniques of free weight matrix, time delay splitting and integral inequality to reduce the conservatism of the stability criterion. The applicable time delay range of the controller gain for system stability guaranteed by this stability criterion is derived by running the MATLAB program, and it is concluded that this paper′s criterion improves the upper bound of the maximum time delay for system stability by 46.33%, which verifies that this paper′s criterion has a lower conservatism. The stability analysis of this paper provides theoretical reference for the analysis and control of more complex servo motor drive systems.