Abstract: The third harmonic conversion of type I/ type II angledetuning scheme of KDP has been studied. And the effects of the walkoff in the third harmonic generation was investigated. A splitstep algorithm based on the Fast Fourier Transform and a RungeKutta integrator is used. As a result, the walkoff decreased not only the tripling efficiency but also the harmonic beams quality. When the waist radius of the fundamental wave is small, the walkoff is the main factor that makes the SHG and THG conversion efficiency decrease, while the effect of detuning angle is little. As the waist radius of the fundamental wave increases, the effect of the walkoff slowly reduces and the effect of detuning angle increases.