We studied a deep convection in the warm sector and abnormal subtropical front caused by a cutoff low over East Asia in June 2009 by using cloud profile radar data from Cloudsat, temperature of black body from FY2C, atmospheric compositions from Aura/MLS, meteorological data from the ECMWF and HYSPLIT4 trajectory model. The analysis shows that for the sake of the background of southern trough, there is a weaker rainfall from the inner fronts, while nearly 1000km-scale deep convection occurs in the warm sector. At the nearly mature stage of the low, the subtropical front induced by the cutoff low is abnormally developing throughout the whole troposphere, which is due to the warm and humid air from southwest gathering along the subtropical front. After rapid eastward movement of the front into the rain belt of the warm sector, the strong ascending branch of thermal indirect secondary circulation across the frontal zone enhances deep convection at the colder part below the front. But it is difficult to sustain because the supplement of water vapor is blocked by the abnormal front. Furthermore, intense stratosphere-troposphere exchange occurs in the abnormal subtropical front zone due to the convective injection into stratospheric intrusions. On the scatter plot of ozone and water vapor these are two special gathering areas with both high (low) concentrations of the two species.