A method to implement the differential precoder for high-speed optical differential quadrature phase-shift keying(DQPSK) modulation is introduced in this paper.This method first researches the optical DQPSK encoder and decoder,and then gets the function of the differential precoder.Depending on the karnaugh maps,we can deduce the logic operations of the differential precoder.Using this method,the logic operations of different DQPSK precoders can be deduced corresponding to different encoders and decoders.