All optical network (AON) needs to rely on all optical logic devices t o realize the transmission of information and the exchange of routing.However,all-optical packet switchi ng technology has not been achieved due to immature all-optical logic devices.The all-optical 4-2priority encoder is proposed based on sum-frequency generation and difference-frequency generation (SFG+DFG ) of periodically poled lithium niobate (PPLN) waveguide to expand new all-optical logic operatio ns.Firstly,the logic expressions of the 4-2priority encoder are analyzed according to the principle of all-optical logic operation based on PPLN waveguide.The high output of the 4-2priority encoder is obtained by using a single PPLN waveguide,and the low output of the 4-2priority encoder is obtain ed by using two cascaded PPLN waveguides.The waveforms and the eye diagrams are obtained by numerical si mulation,and the performance of the 4-2priority encoder is analyzed by the parameters,such as e xtinction ratio,pulse width,full width half maximum (FWHM) and peak power dela y time.The results show that the logic function of 4-2priority encoder can be achieved in optical domain by this conf iguration which can guarantee the quality of the output signals and expand the capability of PPLN wa veguide on the all-optical logic signal processing.