In a system, for the reason of gain saturation of semiconductor optical amplifiers (SOAs) the gain of a channel is influenced by other multiplexing channels, and the interchannel crosstalk limits severely the application of SOA. Theoretical study on channel crosstalk due to gain saturation in SOAs is presented. The system bit-error rate (BER) is quantified as a function of the amplifier input power and the number of channels. It is found that more multiplexing channels induce more severe crosstalk because of the gain saturation characteristic of SOAs. Crosstalk mitigation techniques in SOAs are studied theoretically and experimentally. Simulation shows that continuous wave (CW) light injection can restrain output power fluctuation and decrease BER. With ten multiplexing channels, CW light injection can reduce power loss by 2 dB. Experiment shows in a 40 Gb/s two-channel system, interchannel crosstalk caused by SOA can be restrained by CW light injection.