The traditional method of T/4 delay is easily affectedSbySharmonics. The essay improves the method of T/4 delay. Double T/4 definiteSintegration can filter harmonic of the first vector and pure differential will compensate phase .T/2 definiteSintegration can build a lagging π/2 rad vector as the second. After arithmeticalSoperation of the two vectors, positive and negative sequence separation is completed. Through phase lock loop and rotatingScoordinateSsystem, it gets amplitude of positive sequence. The method is simple and easy to realize especially effective in harmonic suppression. Theoretical analysis and SsimulationSexperiment of Matlab/simulink prove the modified method is correct and advantaged.