Abstract: Methane hydrate phase equilibrium is of great significance in engineering problems related to hydrate formation/decomposition. According to the theory of statistical thermodynamics and CSMHYD software for gas hydrate phase equilibrium, a large number of hydrate phase equilibrium data of temperature, pressure and concentration of NaCl are obtained within the scope of engineering, by which the three dimensional phase equilibrium surface equation is fitted with high precision, reasonable physical meaning and simple expression. The applicable pressure and NaCl solution concentration of the phase equilibrium equation cover almost all the situations possibly encountered in the engineering. The results of the surface equation are in good agreement with the experimental data from a lot of literature. Furthermore, the predicted values of phase equilibrium by the fitted function are more reasonable and accurate in high salinity and high pressure area than that in the literature. Based on the equation, the law of T-p equilibrium line caused by the variation of salt concentration is studied, and Clausius-Clapeyron linear behavior is verified.