[Objective] This paper studied the relationship between rainfall and runoff over upper Nyang river above Gongbo-gyamda Hydrological station to find the critical rainfall that will result to flood. We expected to provide reference for the meteorology pre-warning of mountain torrents. [Methods] Data of ground meteorological observation and CMORPH were used. The upper Nyang river watershed of the hazard critical rainfall was obtained according to the HBV rainfall-runoff model. [Results] After parameter optimization and calibration, the HBV model had a certainty coefficient of 0.91 and NASH efficiency coefficient of 0.89 in the phase of calibration during 2007-2011. Model was validated using the data of 2012-2014, in this phase, certainty factor was 0.86 and NASH efficiency was 0.85. For both the phases of calibration and validating, relative error was averaged as 3.1 %, which implied that the model was applicable. [Conclusion] The flood process in Nyang river basin was not only related to the rainfall in whole basin, but also to the previous water level. The critical rainfall of flood inducement decreased with the increase of previous water level, and vice versa. The critical rainfall showed a nonlinear responsive characteristics.