Radiometer effect and residual gas damping are important interfering forces on the proof mass in purely gravitational orbit, which can affect the construction of the purely gravitational orbit. For spacecrafts in purely gravitational orbit, radiometer effect and residual gas damping describe two different kinds of gas molecules forces caused by temperature gradient and the relative motion of proof mass in the cavity, respectively. These two forces describe gas molecules forces from different views and are both parts of gas molecules forces. However, the coupled model of radiometer effect and residual gas damping can reflect the total force of gas molecules on proof mass. Considering the complex form of the coupled model, the physical parameters in coupled model were analyzed by numerical methods in the Inner-formation Flying System (IFS). These physical parameters contain the relative velocity of the inner-satellite, the inner-satellite radius, the outer-satellite cavity radius, the average temperature and pressure, the temperature difference in the cavity. By fitting a large number of numerical results, an approximate formula for gas molecules forces on the inner-satellite was obtained. Compared with the original results, the fitting errors were less than 20%.