We used simulated data of high spatio-temporal resolution to analyzing the convective instability, conditional symmetric instability, and triggering mechanisms of a torrential rainfall event occurring in Beijing on July 21, 2012. The results indicate that convective instability played the leading role at the initial time of the precipitation. This instability weakened with the occurrence of the heavy rainfall, and conditional symmetric instability was enhanced by the increase in moist baroclinicity and the low-level jet, which maintained and strengthened the subsequent precipitation. Moreover, during the process of precipitation, strong vertical wind shear caused the baroclinic component of the moist potential vorticity anomaly, thus leading to the generation of conditional symmetric instability. Further, during the initial rainfall of the convective instability stage, the terrain lifting force combined with the ascending air on the shear line to boost and stir up the convective instability. In addition, dry air invaded Beijing at the midlevel, which contributed to the convective precipitation. The conditional symmetric instable precipitation resulted in a long-term clash between the cold and warm air over Beijing, which gave rise to the persistent precipitation. The warm air was lifted up by the cold air, which triggered the conditional symmetric instability. Moreover, at 0900 UTC, the wind shifted to an east wind and was abruptly enhanced. The wind was then lifted by the terrain coupled with the ascending air on the shear line, causing intensive upward motion and agitated instability, which is the primary reason for precipitation enhancement during 0900-1300 UTC.