Based on single machine equal area criterion, the stability margin based on single study generator and reference generator was defined.The power angle curve expression for single study machine was analyzed.Based on the Taylor series, the power angle curve of single study generator can be expressed as the function of the rotor angle of the study generator with respect to the reference machine angle, which is fitted by using polynomial function.The stability margins obtained by both prediction method and post event method were compared.Single Generator Equal Area Criterion(SGEAC) based stability margin is the real evaluation of the global stability margin, and it does not need the identification of the two group mode.Lots of example analysis verified the proposed method.