All-optical OR gate is a necessary basic technology in all-optical logic signal processing, and the output optical wave of the OR gate based on periodically poled lithium niobate (PPLN) waveguide in former schemes is mixing wave. In this paper, the all-optical OR gate with single wavelength which consists of three cascaded PPLN waveguides is proposed based on sum-frequency generation and difference-frequency generation (SFG+DFG) in quasi-phase-matching consideration. The signal waveforms and eye-diagrams are obtained by numerical calculation and simulation, and the performance of all-optical OR gate with single wavelength is analyzed by the parameters such as extinction ratio, pulse width and delay time of peak power. The results demonstrate that the logic function of OR gate with single wavelength can be achieved well by this configuration, so that its output optical wave can be used directly in optical domain, thus the connection of optical logical devices can be improved, the speed of processing can be increased, and the critical foundation of developing new all-optical logical devices can be provided.