This paper presents a theoretical model for evaluating the standard deviation of Four-Wave-Mixing（FWM）noise in multi-span dispersion managed WDM systems with Differential Phase-Shift-Keying（DPSK）modulation.This model takes into consideration the effects of the randomness of bit sequence and initial phase of the pulses in each channel.Using this model

it analyzes some practical cases.The results show that with the same average power

the noise deviation ratio of DPSK modulated signals is smaller than that of On-Off Keying（OOK）modulated signals

that the variations of bit rates have little effect on the noise deviation value of the DPSK modulated signals

while the average power has great influence.With the increase of the latter

the voice deviation value increases almost exponentially.