A theoretical analysis is presented for conversion of third-harmonic generation (THG) in KDP crystal for type Ⅰ/Ⅱ angle-detuning scheme of high-intensity laser. Taking into account the the third-order nonlinear effects, transverse walk-off and diffraction, mostly the effects of third-order nonlinear χ(3) interactions on the frequency conversion and the phase matching condition are discussed. A split-step algorithm based on the fast Fourier transform (FFT) and a Runge-Kutta (R-K) integrator is used. The results show that the third-order nonlinear effect decreases the tripling efficiencies and increases the modulate depth of the output intensity of 3ω radiation. However, adjusting the angular detuning (Δθs) can avoid effectively the third-order nonlinear effects.