In this paper, analytical and numerical methods are used t o study cross_phase modulational instability. Firstly, by use of coup led nonlinear Schrdinger equation, we have studied the cross_phase modulation i nstability in the normal and anomalous dispersion fiber links of wavelength divi sion multiplexed system with an active amplifier, when the amplitude is perturbed. Se condly, m aking use of the split step Fourier transform and Monte_Carlo method, we simulat ed the cross_phase modulation instability in active normal and anomalous dispers ion fibers with fluctuation of phase noise. Comparison between the above met hods shows that the results agree with each other.