To clarify the current situation of water resources reserved for exceptional drought in the water network area is the premise of improving the regional water network pattern and enhancing regional drought resistance. Aiming at the problems of unclear concept of water source reserve and lack of calculation method of reserve scale, this paper puts forward the definition of water source reserved for exceptional drought on the basis of analyzing existing concepts of water source reserve, and gives the calculation method of water source reserve scale for four types of water sources: lake and reservoir water, river water, groundwater and external water. The method was applied to the intake area of the middle route of the South-to-North Water Transfer project. The results showed that the total water reserved for exceptional drought of the intake area reached 12.252 billion m3, among which the water reserved by lake and reservoir, river, groundwater and transferred water accounted for 10.86%, 15.7%, 56.31% and 17.12% respectively. Among the 20 cities in the water receiving area, only 7 cities have the above 4 types of water sources in their water reserves to cope with severe drought, and 13 cities have groundwater reserves that account for more than 50%, and the type of water reserves is single. There are four cities with small reserve amount and single reserve type, so it is urgent to further improve the water resources reserve system for exceptional drought in the intake area of the middle route of the South-to-North Water Transfer project.