With the development of the internet, network data traffic is increasing rapidly, the concept of all-optical network (AON) is helpful to solve the problems of limited available bandwidth and low signal transmission rate. Among them, all-optical logic signal processing is an important part of AON. According to the principle of second order nonlinear effect of periodically poled lithium niobate(PPLN) waveguide, the output signal is successively sent to different PPLN waveguides for superposition processing, and the waveguide cascade structure of all-optical 2-4 line decoder is designed. The waveforms and eye diagrams are obtained by numerical calculation and simulation, and the full width half maximum (FWHM), peak power, delay time, extinction ratio of the output signal of all-optical 2-4 line decoder are analyzed. The results show that the logic function of 2-4 line decoder in optical domain is realized by cascading PPLN waveguides, and the transmission quality of optical signal is guaranteed, which provides a new device type for all-optical signal processing of PPLN waveguide.