A generalized model to evaluate variance of four-wave-mixing (FWM) noise in dispersion managed wavelength-division-multiplexing (WDM) system with intensity modulation scheme is presented. The information sequence of bits and initial phase of bit sequences are random. The model gives the statistical results of them. Simultaneously, the positions of inter-channel pulse collisions are also included in the model, which are more difficult to be located in dispersion managed fiber link than in constant dispersion fiber link due to inter-channel pulse walk-off. Some examples show that optimized dispersion management schemes are important to decrease FWM noise, and meanwhile make pulses transmit steadily. It is also shown that when the pulse duty cycle is small, initial relative time delay of bit sequence plays an important role in obtaining minimized value of FWM noise variance.