A method for solving the inverse problem of optical parametric chirped pulse amplification, i.e., for a given shape of the output signal pulse, how to calculate the shape of the input signal pulse, is proposed. Based on numerical techniques such as split-step Fourier transform and fourth-order Runge-Kutta, the quantitative relation between the input signal intensity and the output signal intensity was obtained by numerical fitting. Taking the chirped Gaussian pulse and the shaping pulse with a certain shape as the required output signal pulse, the input signal pulse shape can be worked out by the inverse calculation. The results indicate that the inverse problem can be solved quickly and precisely with the method proposed in this paper, and this method is useful for pulse shaping.