Super Typhoon Muifa (1109) got enhanced and weakened twice during its 12 d lifetime. The intensity was predicted stronger during 3-7 August because the high SST of Kuroshio was considered. In this paper, comprehensive and quantitative diagnostic analyses on main factors affecting typhoon intensity are performed. The results suggest that though the ocean thermal conditions are certainly favorable for the typhoon, the ambient field conditions, both the cold air advection and the ambient vertical wind shear, are unfavorable. The negative feedback which well matches the process is the main cause of the weakening.