In this study, based on the daily rainfall data of Beijing from 1984 to 2013, the Forestry Building and the surrounding lawns in the main campus of Beijing Forestry University are choosen as the study area, three types of rainwater usage scenarios(i. e. , flushing toilets inside the building, irrigating lawns around the building and combination of flushing the toilets and irrigating the lawns)are set, the continuous simulation approach is applied to draw the variation curves of rainfall interception rate, tap water replacement rate, probability of water supply and cost-benefit rate for these rainwater collection and utilization projects under the said three kinds of rainwater usage scenarios. Then the optimal design capacity(60~82 m³)of the rainwater storage reservoir is determined by comprehensively analyzing the change features of these curves. Meanwhile, the design storm approach is also applied to calculate the design storage capacities of the reservoir for 1~5 years of different recurrence interval. Comparison of the two design approaches shows that the continuous simulation approach can be used to comprehensively measure stormwater management effects, effects of relieving water resources shortage, water supply reliability as well as economic feasibility analysis of these rainwater collection and utilization projects, though its computation process is relatively more complex.