Abstract: In this article,the conversion efficiency of the third harmonic generation induced by second-order cascading in nonlinear photonic crystals is analysed when quasi-phase matching is satisfied. The numerical calculation shows that the conversion efficiency of third harmonic generation is strongly modulated by second harmonic signal in input light. If the ratio of the intensity and the primary phase of second harmonic signal and fundamental light satisfy certain condition,a high conversion efficiency of the third harmonic generation can be found in a wide scale of input intensity.