Whether the assumption of the same frequency of storm and flood is true or not and the probability distribution of encountering the storm and flood are the basis for the design flood calculation of small and medium-sized basins. To validate the rationality, based on the hourly rainstorm and flood observation data from 1967 to 2013 in the typical heavy rainfall basin of Caojiang River in the upper reaches of Jianjiang River, this paper firstly analyzes the marginal distribution of the maximum surface rainfall and the maximum flood peak discharge, then analyzes the encounter probability of the joint distribution of rainstorm and flood peak, and calculates the design recurrence level. The main conclusions are as follows: the probability of the same frequency of rainstorm and flood is small in this basin; for a specific design frequency, the encounter probability of flood peak discharge and watershed surface rainfall increases with the increment of watershed surface rainfall; the encounter probabilities of rainfall and flood peak flow show that there are many management options for flood risk; compared with the ‘OR’ return period and the ‘AND’ return period, the secondary return period more accurately reflects the risk rate of R-Q combination.