It rained from Jul 3rd to Jul 6th, 2000, last 72 hours. The total rainfall reached 358. 19 mm. By continuous monitoring discharge of the "way out" section in the small basin, analysized the rainstorm characters, based on which, further analysized the connection and development course of catastrophic rainstorm and flash flood. The results were that: the rain belonged to catastrophic rainstorm, the characters were local, long and intensive. The max rain intensity was 32. 84 mm/h, appeared in later stage of the rain. Flash flood in "way out" section appeared in the later-middle stage of the rain and was especially evident in later stage. In addition, the drainage area was smaller and the confluence time was shorter, so flash flood came earlier. At the same time, another two results were thatrthe max flood peak discharge during raining was 1.185 m3/s, the runoff modulus of max flood peak discharge was 3. 66 m3/(s km2), the runoff coefficient of max flash flood in calculation interval was 0. 567. The total flood volume come from the rain was 33 030. 54 m3, average runoff coefficient was 0. 304.