The cause of rainstorm and flood is complex, with its randomness and certainty. During the actual design process of flood control and dispatching, however, the randomness of flood was emphasized and certainty was ignored, which resulted in that the design divorced from practice and flood resoures could not be fully used. In order to solve this problem, this paper offered a flood classification method based on storm genesis with the relatively stability of weather system and certainty of flood process being considered. Dahuofang Reservoir as an example has been used to state the flood classification method. Through flood process graph and similarity coefficient, the rationality of classification can be proved. Flood classification discussed in this paper will have great significance in understanding flood rules, increasing flood capacity, adding flood utilization, and researching flood control clas- sified dispatching mode design.