Typhoon Meranti intensified significantly after moving into the Taiwan Straits. This paper performed a survey on the intensity change of tropical cyclones with a similar track to Meranti in history and found that almost no TCs intensified. The process of the intensity change of Meranti in the Taiwan Straits is analyzed. We find that the intensity estimates provided by many typhoon forecast centers in operation are weaker than observations greatly. By using synoptic and dynamic diagnosis, we analyze the basic environment conditions of the tropical cyclone’s development when Meranti moves into the Taiwan Straits and find that the high SST in the Taiwan Straits, the abundant water vapor transport in lower and middle levels, the strong low level convergence and upper level divergence, and the intense low level positive vorticity transport provides the favorable dynamic and thermodynamic conditions for the intensification of Meranti, and the weak vertical environmental wind shear cannot inhibit the development of Meranti. These results may turn out to be meaningful references for operational TC intensity forecast.