Abstract: Based on some numerical techniques such as split-step Fourier transformation,fourth-order Runge-Kutta and iteration method,the method for solving the inverse problem of monochromatic third-harmonic generation,i.e.the fundamental field is calculated from the given third-harmonic field,is proposed.The numerical calculation is performed and analyzed.The effects of the initial fields on the time and the precision of the iterative calculation are also discussed.It can be shown that our method is available and efficient.