The tasks of the design calculation of rainwater catchment system for life purposes are determining the optimum rate area of rainwater catching field and the first rate volume of water cellar, which are the minimum area of rainwater catching field and the minimum volume of water cellar which can meet the needs of supplying life with water. Taking the characteristic hydrologic year that its frequency is 75% and 95% as the basic calculating years, and taking the precipitations of rainfall periods, the days of non rainfall periods, the days of the maximum non rainfall period and the index of per capita water need as the parameters, according to the precipitation data day by day, the first rate volumes of water cellars and the first rate areas of rainwater catching fields in the mountain areas of west He'nan province are calculated.