Abstract: The paper calculates the viscosity coefficient of two-phase solution that contains gas and liquid with the statistical mechanics and dynamics method. It is considered that the method is more intutive and simple than common liquid theoretical method to analyze and accurately calculate the viscosity of two-phase flow versus its temperature, flowing pressure and void fraction. It is found that the viscosity is related to temperature void fraction and pressure, that is, the pressure will strongly affect viscosity as void fraction increases. The coefficients of water and dilute solution containing gas are calculated, and it shows that the viscosity declines as dillute solution increases; if the flowing pressure raises, the viscosity also raises. The calculated results are compared with the experimental results of heavy water and dilute solution. The results show that the error of theoretical results is less than 10 %.