For power systems containing large number of power electronic converters，when the detailed model of switch circuit such as pulse width modulation（PWM）converter is applied，transient simulations using conventional EMTP method are very time-consuming.An improved EMTP algorithm is proposed to perform simulation with the piecewise averaged model of PWM converters.The proposed algorithm is able to avoid the complicated procedure applied to handle the switching events，while two new sub-procedures are included to predict the parameters of the piecewise averaged model and adjust the errors caused by the inaccuracy of parameters are added.From the test results of three-phase PWM AC-DC converters，it can be seen that by applying the proposed algorithm，the efficiency of transient simulations can be enhanced remarkably.