Super typhoon Saomai (2006), which struck China in August 2006 as the most intense typhoon China had experienced in recent 50 years, is a typical rapid-intensification tropical cyclone over the coastal water of China, whose intensity is even stronger than hurricane Katrina (2005) which is the costliest and one of the deadliest hurricanes in the history of the United States. Saomai is estimated to have been responsible for nearly RMB 20 billion yuan in damage. It can be traced back to a tropical storm on 5 August. The system strengthened into typhoon 2 days later and rapidly intensified into super typhoon over the coastal water of China on 9 August as the central pressure fell to 920 hPa. It then maintained its peak intensity for about 18 h before beginning to weaken. Saomai tracked into Zhejiang Province in the afternoon on 10 August, making landfall in Cangnan County and then rapidly weakened. Taking super typhoon Saomai as an example, a case numerical simulation study is made for different developing stages by the use of non-hydrostatic version 3.5 of the fifth-generation Pennsylvania State University-National Center for Atmospheric Research (PSU-NCAR) Mesoscale Model (MM5). The model can well simulate the track and intensity of typhoon Saomai, and can basically capture the characteristics of intensity change of Saomai, especially its intensification over the coastal water of China. The results show that the intensity change of Saomai is in inverse proportion to that of the South Asia high and the subtropical high, the South Asia high and the subtropical high decay in the typhoon rapid intensification stage. There is a favorable environment for typhoon intensification, such as strong divergence at the upper level, sufficient water vapor transportation and the merge of adjacent vorticity lumps. And the addition of divergence at the low-middle layer and convergence at the middle layer are associated with typhoon weakening. There is a direct relationship between the condensation latent heat and intensity of typhoon Saomai, the enhancement of the latent heat in the mid-upper troposphere is favorable to Saomai's intensification. The sensitivity experiments of orographic effect are carried on to study their effects on the change of intensity and structure of Saomai by keeping/removing mountains in the eastern area of Zhejiang and Fujian provinces. Contrast experiments demonstrate that small terrain has a little impact on track or intensity change of Saomai, but a large one plays an important role. Exactly, the general circulation would be changed by a large terrain.