Based on the recording precipitation data in 1980-2002 at 12 meteorological stations over Hehuang valley of Qinghai Province, the spatial characteristic and its instability of different time as well as design heavy rainfall in various time interval and frequency over the valley is analyzed. The result shows that the maximum rainfall in various time interval over the valley appears increasing. With the delaying of precipitation time and increasing of rainfall, its tendency rate has been increasing while the periodic wave of interannual variation is becoming slowly, and the spatial distribution characteristic of maximum rainfall's average in each time interval is getting close to spatial distribution of annual precipitation, and topography has significant impact on the spatial distribution of the heavy rainfall. The maximum rainfall in 30 minutes is extremely instable. In the meantime, with the delaying of precipitation time interval, the stability of maximum rainfall is inclined to increasing. The distribution of maximum rainfall in each time interval appears skew, and the spatial distribution of maximum rainfall of different frequency in each time interval is generally consistent with observational data, but less instable than it.