Considering two-layer fluid bounded by the thermocline, under the condition of weak nonlinearity, based on the dynamic equations of two-layer fliud, the nonlinear Schrdinger equation for internal waves in deep sea is obtained by adopting hierarchical method. The coefficients of dispersion and nonlinearity are analysed, the soliton solution of the nonlinear Schrdinger equation is obtained, and the exactitude of the theory is demonstrated by a numerical experiment.