Cumulus merging in two severe convective systems occurred in Henna and Anhui on June 14, 2009 was analyzed based on radar data and data of LAPS(Local Analysis and Prediction System). The results showed that both cell merger and convective system merger enhanced obviously and developed into severe convective weather. In Anhui region, the development of the mesoscale convective system included clouds merger from small cell to large cell or cloud cluster, then to small convection system, at last, to large convection system. The severe convective weather was stimulated by gust front , which growed to severest during merging. Both the air flow from the low cells and environmental steering wind played important role in merging process, the interaction between clouds circulation and environmental steering wind was responsible for cumulus merging. The results of LAPS showed that stratification instability, mesoscale convergence and vertical wind shear were the main conditions for development of cumulus merging and severe convective system.