The paper advances a new calculation method improved by using(2~5)/1.2rm rule which concerns the influence of the fluctuation of molecules on the results of molecular dynamics simulation of the interface.The densities of saturated vapor and liquid of homogeneous phase and the surface tension of inhomogeneous phase of CH4 are obtained by applying this new calculation method.Compared with the traditional molecular dynamics simulation,this method has less error in calculating the surface tension of inhomogeneous phase and takes less calculating time.