The high conversion efficiency of coupled third harmonic generation (CTHG) is achieved in the non-periodic optical superlattice (NOS) which is devised by the nonlinear conjugate-gradient (NCG) method. For either the single wavelength case or the multiple wavelengths case, the conversion efficiency of CTHG is higher in the NOS structure than that in the aperiodic optical supperlattice (AOS) which is devised by the traditional simulated annealing method. Since the thickness of each domain is arbitrary and further optimization can be done by using NCG method, it is easier to achieve the optimal quasi-phase matching (QPM) in the NOS structure. The variation of conversion efficiency as a function of the optical wave propagating distance is calculated, and the results show that the conversion efficiency increases with the increase of the optical wave propagating distance, which indicates that the contribution of every domain on CTHG process is positive, and that the optical QPM can be achieved in the NOS structure.