Abstract: To solve the heavy hydrocarbon precipitation problems in LNG industry, the theoretical model of three phase equilibrium is established and the thermodynamic calculation software is developed independently. The interaction parameter K_ij in calculation model represents all the physical properties changing when the pure substance becomes mixture, and it's the function of T, x and component properties. It can't be calculated by a formula as physical chemistry changes of the mixing process have not been understood comprehensively now. The phase equilibrium in different K_ij is calculated in order to select the appropriate K_ij to improve the accuracy of the model calculation results.