Theoretical research on the cascaded second-order nonlinearity based on periodically poled lithium niobate waveguide in pulsed pumping is presented. By numerical simulations the propagation figure is obtained and it exhibits the waveform evolution of pump, second harmonic, signal and difference frequency pulse. The analyses demonstrate that harmonic pulse is continually broadened along propagation length because the walk-off occurs among four pluses due to group-velocity-mismatch, and then converted-pulse-waveform distorts with peak offset. The wavelength conversion experiment has been completed in pulsed pumping and converted-pulse-waveform distortion has been demonstrated.