Communication signal delays result in the system dynamics model with tim-delay and an AGC system can become unstable. The quantitative analysis of delay margin is the key problem to be solved. A linear time-invariant（LTI） system with commensurate delays is modeled and a method to compute the delay margin of AGC system is proposed based on the state spaces analysis method and matrix analysis theory. Simulation results of the East China Power Grid validate the methodology. This work is supported by the National Natural Science Foundation of China（No. 50595411） and Program for New Century Excellent Talents in University.