Using the mesoscale regional model GRAPES_Meso, this paper simulates a frontal rain band which occurred in North China during August 2-4 2015. The simulated results are in good agreement with the observations. The convective stability, inertial stability and conditionally symmetric instability (CSI) during the precipitation process are analyzed using the model output data, and the CSI region is diagnosed. The results show that: (1) The development of CSI was accompanied by the weakening of convective instability and the enhancement of inertial instability. (2) According to the change of instability, the precipitation is divided into three stages. In the first stage, the precipitation was mainly affected by convective instability; in the second stage, convective instability, inertial instability and CSI got enhanced, and precipitation at this stage was affected by three types of instability; in the third stage, the three types of instability gradually weakened, but still affected the sustainability of precipitation. (3) In the vigorous developing stage, CSI showed a bandlike distribution in the plane. In the profile, the CSI is active in the lower troposphere. (4) In diagnosing CSI, the method with MPV and convective instability is more effective than the one with M-θe.