Abstract: This paper studied the possible mechanism for the deformation field affecting the occurrence and development of torrential rainfall, from the perspective of the frontogenesis and slantwise vorticity development. Diagnostic study of the "7.21" severe rainfall case which occurred in Beijing on 21 July 2012 showed that the deformation tick marks had the same orientations or a small angles with the moist potential temperature contours when the rainfall presented in Beijing, which implied the local frontogenesis. The frontogenesis function calculation results showed that the deformation term of the frontogenesis function had a significant contribution to the low-level frontogenesis which induced torrential precipitation in Beijing. The high-level jet stream then accelerated and veered northernly, which resulted in strong high-level divergence and enhancement of severe precipitation in Beijing. The diagnostic results also showed that the vertical vorticity had an explosive growth accompanying the occurrence of severe precipitation. Moreover, in the Beijing district, changes in the baroclinicity of the atmosphere had a similar growth trend with the vertical vorticity. According to the slantwise vorticity development theory, the baroclinicity term associated with the deformation in the complete form of vertical vorticity equation was analyzed. The results showed that the deformation term driving vertical vorticity had positive values in the vertical vorticity growth areas, which implied that the deformation had a significant contribution in the development of vertical vorticity in the "7.21" severe precipitation case. In view of the deformation drove air contraction in its confluence axis, the moisture flux deformation was introduced to include moisture into the deformation. It was shown that the low-level positive anomalies of moisture flux deformation corresponded well with the rainfall areas. The moisture flux deformation contained two parts:one was specific humidity advection and the other was deformation. The deformation determined the basic distribution of moisture flux deformation while the specific humidity advection had more rainfall forecasting implication.