The dual-tripler scheme of KDP crystals for third harmonic generation (THG) is studied by the equations of three interacting waves under the small signal and the high power (1.5 GW/cm2) condition. The 8 mm type II KDP tripler with an detuning angle of 0.35 mrad whereby the addition of a 6 mm type II tripler with an detuning angle of -0.25 mrad can increase the bandwidth of third harmonic generation to 0.3 nm at 1.5 GW/cm2. Meanwhile, the conversion efficiency of the third harmonic generation is closely related to the distance between two triplers. If the relative phase among the three interacting waves changes π during the distance, the third harmonics from the first tripler can return to the second harmonics and the fundamental frequency and the conversion efficiency will drop by a wide margin. To ensure the optimum performance of the dual-tripler system, it is necessary to find the correct air gap during which the relative phase among the three interacting waves changes 2π.