In rainstorm system,  mass of wet air can be decreased by heavy precipitation,  and this decrease introduces a force on the development of rainstorm and this force is called "mass forcing" in this article. First,  the tendency equation of perturbation pressure for ARPS numerical model,  which includes mass forcing,  is deduced by introducing a modified continuity equation. Secondly,  with this model,  a cold-front rainstorm in North China is simulated and diagnosed. Result shows: (1)Mass forcing can influence the amount of precipitation. For this rainstorm process,  the maximum mass forcing shows up in the height of 5750 m where both the water vapor condensation and pressure declination are strong,  also it produces both upward and downward movement of the air flow. Furthermore,  the downward movement suppresses the rising movement of lower inflow,  which decreases the amount of precipitation and downgrades the rainstorm. (2)The distribution of mass forcing is coincide with that of cloud water content. The change of mass force agrees with that of precipitation rate,  especially in the height of 5750 m. The terminal falling speed of hydrometeor places the biggest influence on the mass forcing. (3)The change of local pressure field caused by change of wet air mass field can result in adjustment of dynamic field,  such as convergent,  divergent and vertical movement. Reciprocally,  the change of dynamic field can also affect the pressure field. So this interacting process will yield visible effect on the intensity of rainstorm. (4)The dominating terms which lead to different local perturbation pressuress are the difference of convergent term and that of vertical term,  while that mass forcing has an indirect effect on it. The leading term which controls the distribution of local water vapor and water material is the difference of microphysical term,  followed by the terminal falling speed of hydrometeor.