Based on the NCEP/NCAR reanalysis data, the characteristics of Rammasun over the offshore of China are analyzed using the synoptic and dynamic diagnosis methods. The results show that high SST in the South China Sea and the abundant water vapor transport at lower and middle levels provided favorable energy for the intensification of Rammasun; weak vertical environmental wind shear between 0 to 4 ms-1 at lower, middle, and upper levels, which maintained nearly 22 hour, was a necessary condition for supertyphoon Rammasun landing two times; strong divergence at the upper level, positive vorticity which enlarged at the lower level, and the positive vorticity column stretched to the upper troposphere resulted in Rammasun increased sharply; there was a time lag of about 12 hours between the vertical wind shear change and the typhoon’s rapid intensity change; and the changing of vorticity at the lower troposphere was also about 9 hours earlier than typhoon strengthening.