According to radar observations and sounding data in Guiyang in 5th in May in 2005, combined with the results of numerical simulation with WRF model, the processes of merging and contributions to precipitation are studied in detail, especially the changes before and after merging. The analysis shows that the shear line and the vortex have an important role in merging. The whole processes experience multi-scale merging processes from single-cell scale merging to cloud cluster-scale merging, and high core merging. Besides, new convection merges into the system continually and keep the system develop. We study the main parameters of the merging process.