In order to encrypt multi-wave input signals simultaneously,according to the analysis of the effect of light interference,we conclude that XOR encrypting operation can be realized by the interference of two coherent optical signals on e of which is given pi phase shift before they combining.According to this concl usion,an all-optical encryption system which can be used in wavelength division multiple xing (WDM) system and encrypting multi-wavelength optical signals is proposed.This scheme achieves multi-wave l ength coherent optical signals and optical keys by direct electro-optics modulation ,and achieves pi phase shift of multi-wavelength optical keys by phase shifter,then the XOR encrypting operation for optical signals can be realized by the combining and interferring of optical signal and key which have the same wavelength,and then t he multi-wavelength encrypted result is output.The feasibility and effectivenes s of this system are confirmed by the simulation of encrypting operation of 8×10Gbit/ps WDM signal s.The bit error rate (BER) and Q factor of encrypting output are 0and 164,resp ectively.Moreover,this paper analyzes the Q factors of e ncryption output at different duty cycles of input signals.When the duty cycle o f input signal is smaller than 0.43,the Q factor of encr ypted result is about 164.