Using NCEP re-analysis data and multi-observation data from meteorological satellites, weather radar, and a surface observation net, we analyze a rainstorm convective cloud merger case on 22 July 2008 in the Huaihe River basin. This cumulus merging event was a typical multi-scale (including cell merger and cloud core merger) and multi-formation merging process, and was caused by a low-layer atmospheric pressure gradient in the unstable region. The entire merging process could be classified into three stages: cell development, cloud bridge formation, and system merging. As a result of the merger, large, complex convective systems were produced and became more intense, and heavy rains were recorded at the surface. After cloud cores merged, the cloud area increased. Satellite cloud images revealed that the cloud boundary became smoother, and increases in the echo top and vertical integer liquid could be observed in the radar echoes. After the merger was complete, the highest cloud top was descended[H4], and the vertical integer liquids were in a downdraft, which caused heavy, expanded ranges and long-duration precipitation on the ground. Three dynamic factors caused the clouds to merge: First, the horizontal asymmetry of vertical movement in the large-scale environment field was an environmental factor. Second, the surface pressure gradient force of the atmosphere was one of the dominant factors for cloud bridge generation and cloud merging. Third, the main reason the cloud cores merged was the coupling of vertical motion between one core’s downdrafts and another’s updrafts.