In this paper, we simulate a process of binary typhoon and the budget diagnoses of water vapor in this process to analyze the transportation characteristics of water vapor and their influences on the variation of typhoon intensity. The results show that the interactions between typhoon Fitow and surrounding systems, including subtropical high, mid-latitude trough, west of the continent high and Southeast trailing typhoon Danas, change the background wind fields of Fitow, and then adjust the transport channels of moisture. Those surrounding systems, especially the trailing typhoon Danas which can be called the collection-transfer station of water vapor, have important effects on the intensity maintenance and the northern strong precipitation in the offshore and landing period of Fitow. The distribution and evolution of water vapor flux convergence band are consistent with those of strong convection band, revealing that the water vapor transport has important influences on the structure and intensity of the inner-core convection band in typhoon. The budget results show that the time series of total water vapor flux and typhoon intensity change synchronously. And the eastern boundary is the main source of water vapor transport, and the southern and northern boundary are also important, while the western boundary makes a negative contribution. The inflow transport channel is mainly located at the bottom of the troposphere, while the outflow transport area of water vapor is located at middle- and low-level troposphere of western boundary. The vertical transportation of water vapor plays an important role in redistributing the internal moisture of typhoon. The duration of sever convection band in typhoon is accompanied by the strong vertical transport of water vapor, which indicates that the vertical transport of water vapor is important for developing the strong convection in U and V type typhoon.