Physical characteristics evolution in cloud merger is critical to the evolution of cloud precipitation, which is also important for short-term forecasting and study of precipitation. So, in this paper, using WMF (Weather Research and Forecasting) model, the authors simulated a merging process in Guiyang on 5 May 2005, combined radar observations and MICAPS (Meteorological Information Comprehensive Analysis and Processing System) data, and analyzed the processes of precipitation convection merger and the evolution of associated physical characteristics. The cumulus merger processes are of diversification, experience the process from neighbouring isolated cells merging to cell-cloud cluster merging, and neighbouring cloud clusters merging into widespread precipitation system. In the merging processes of several stages, the change of temperature stratification structure curve all shows that unstable energy releases during the merging, and unstable stratification before merging tends to more stable stratification after merging, especially at middle levels, tends to neutral one, which is advantageous for the dominant development scale becoming longer. The physical characteristics alter significantly, which shows that before and after the first merging and complete merging, the maximum precipitation at the convective center all increases strongly. However, the changing of the maximum upward flow velocity is contrary to the changing of the maximum precipitation. The maximum upward flow velocity generally decreases after the merging. While, the average upward flow velocity all decreases before and after the first merging and complete merging. The average precipitation in the whole cloud region mostly tends to increase.