According to the applicability conditions for three type of internal solitary wave theories including KdV, eKdV and MCC, and integrating with fully nonlinear Euler equations, a numerical wave-generating method for internal solitary waves in two-layer fluid was proposed by adding mass sources  to continuity equation. Series of numerical simulations were presented to analyze the generation mechanism for the internal solitary wave in the process where mass sources release and absorb fluids. Besides, the variation characteristics of the waveform, amplitude, and flow field induced by the internal solitary wave were discussed. The results show that the waveform remains stable and the amplitude decays much weakly during the propagation of the internal solitary wave generated by the present method, and its waveform and amplitude are in good agreements with the experimental and theoretical results.