The interband crosstalk in the WDM system will degrade the system BER by penalizing the extinction ratio in each channel. But in multi- wavelength optical cross-connect networks, there is another kind of crosstalk, intraband crosstalk, in addition to the interband crosstalk. The intraband crosstalk will result in beat-noise which can be detected by the receiver and has more severe impact on the system BER than interband crosstalk. In this paper, we analyze two kinds of crosstalks, and show that our calculat result coincide with the results from experiment very well.