Under the background of global warming,research on the meteorology disasters of small and middle rivers in Qinghai Province can help us conduct disaster forecast and loss assessment.A case study of Longwu River Basin indicated that HBV model well simulated the process of flood response to rainfall after model calibration by using daily meteorological and hydrological data for the period of 2001-2011,while the NASH reached 0.69 and 0.83 in verification period and calibrated period.The model combined with stage-discharge relationship was then used to calculate the dynamic critical rainfall over the Longwu River sub-region,and the high correlativity of runoff and water levels was discovered.The critical rainfall decreases with the increase in former water level.The variation of critical rainfall with different former water level showed significant nonlinear response characteristics in order to reduce losses caused by flooding disasters.