In third harmonic generation, it is v ery important to select the parameters of the generator. In this paper, we optim ized the thickness of the doublers and triplers, mismatch angles and other param eters of Ⅰ／Ⅱ angledetuning and Ⅱ／Ⅱ polarizationmismatch third harmonic generation schemes by numerical calculation way. We also give the tripling effi ciency curves which the pulse shape of the input intensity is flat in space and time around 5.0 GW/cm2.