A numerical simulation on wavelength conversion based on a periodically poled lithium niobate waveguide is presented. The evolution of pulses is exhibited and the walk-off between pulses is discussed to explore the influence of two different injecting modes that the pump wave and signal wave are injected from the same direction or from the reverse direction. The analysis results demonstrate that the second harmonic generation and the difference frequency generation are simultaneously realized in the same-direction-injected wavelength conversion, which widens the second-harmonic wave continuously because of the walk-off among the reactive pluses due to group-velocity-mismatch, and then the converted pulses produce waveform distortion in the subsequent difference frequency generation. The reverse-direction-injected wavelength conversion is proposed to avoid converted waveform distortion, and it is able to separate two processes of the second order nonlinear effect. It is expected to amend the waveform distortion of converted pulses as well as to improve the conversion efficiency.