Building a rainwater system based on the idea of low-impact development (LID) is an important aspect of the current “sponge city” construction in China. The “sponge city” concept emphasizes that the runoff can permeate the soil or be stored temporarily, and rainwater could be used again when it is needed. Beijing is one of the earliest cities to study rainwater harvesting and LID techniques in China. Through long-term monitoring of rainfall, runoff flow, and water quality of a campus demonstration project in Beijing, the runoff quantity and pollutant concentration variations have been analyzed. Furthermore, the runoff reduction effects of single LID measure, such as green roof, filtration chamber, and permeable pavement, have been investigated. Additionally, the overall reduction effectiveness of the LID system on the average annual rainfall runoff and pollution load has been discussed. Preliminary studies suggest that runoff pollutant concentration is positively correlated with the rainfall interval time, and the longer rainfall interval time leads to higher runoff pollutant concentrations. The very good outflow quality of the rainwater harvesting system could satisfy the reclaimed water quality standard for scenic entertainment use. The non-point-source pollution reduction effects of the LID system are obvious because the pollutants could be removed by filtration on the one hand and the pollution load could be reduced because of the significantly reduced outflow on the other hand.