In order to acquire the largest extinction ratio (ER) of the converted signal in the cross gain modulation (XGM) wavelength conversion, the signal wavelength λ pk,s should be longer than the small signal gain peak wavelength of the semiconductor optical amplifier (SOA). The influence of the signal power, the signal ER, the probe power, the probe wavelength and the drive current of the SOA on λ pk,s  and the improvement of the ER is studied in detail by using the multisection model, and considering the nonsymmetry of the gain profile and the shift of the gain peak wavelength along with the carrier density. It is shown by numerical simulation that λ pk,s should shift to the longer wavelength about 7 nm when the signal power has a increment of 3 dB, while λ pk,s should shift to the shorter wavelength when the signal extinction ratio, the probe power and the drive current of the SOA is increased. The improvement of the ER is enlarged remarkably with the increment of the signal power and the SOA drive current, but decreases with the increase of the signal ER. The change of the probe power and the wavelength has little influence on the improvement of the ER.