A theoretical analysis is presented for third harmonic conversion in KDP (KH2PO4) for type Ⅰ/ type Ⅱ angle-detuning scheme of high-intensity laser. Taking into account the third-order nonlinear, transverse walk-off and diffraction, the conversion efficiency of the third harmonic has been calculated. The results show that the efficiency decreases with the phase ripples of the fundamental wave, and the higher of the THG conversion efficiency is, the smaller of the modulate depth of the output intensity of 3ω radiation is, which indicates that by increasing the conversion efficiency the beam quality of the third harmonic can be improved.