The dual-tripler scheme for third harmonic generation (THG) is studied by the coupled equations under the small signal and high power condition (1.5 GW/cm2). The design has been proved in the experiment. It found that this design can increase the third harmonic conversion efficiency of broad bandwidth, but the distance between two triplers has an important impact on the third harmonic conversion efficiency in the experiment. 19.75% THG efficiency with chirp pulse of 3.5 nm bandwidth at average power of 1.5 GW/cm2 was achieved at 25 mm optimum distance between two triples in the experiment. It will be useful to solving the problem of the large bandwidth of the THG in the high power laser system.