An amplitude of internal solitary wave as high as 100 m has been reported in the South China Sea(SCS). Large-amplitude internal waves may cause severe damage to the ocean engineering and submarine. The fully-nonlinear Schrdinger equation (FNLS) that describes large-amplitude internal-wave propagation in deep sea is derived from the governing equations of hydrodynamics by introducing the perturbation expansions with the assumption that the fluid is in a two-layer stratification system. This is a theoretical model for internal-wave dynamics, which is different from the well-known KdV series formula and BO equation. Moreover, a numerical simulation is carried out in the SCS. The results show that waveform and group velocity in evolution are in accordance with the MODIS images, so the model is suitable for simulating the large-amplitude internal-wave behavior in deep sea.