The delay margin is defined as the maximum time delay that a power system can sustain without losing its small signal stability. The estimation of delay margin is important in the application of wide area measurement system（WAMS） data and evaluation of wide area controllers. Firstly， the single time-delay model of power system is introduced. Then， a simple and effective method to determine the delay stability margin is proposed. The transcendental characteristic equation is converted into polynomial equations at the imaginary axis without any substitutions. The delay margin of a single-machine infinite-bus system at typical equilibrium point is obtained through this method. The effect of exciter parameters on the system delay margin is also discussed. At last， the validity of this method is verified with the time domain simulation results.