In this study, water sampling was carried out extensively through collection of natural rainfall and municipal surface runoff water samples in the southeast region of Beijing Municipality during the period of June 2002 to July 2004. Chemical tests and measurements of collected water samples were conducted in the laboratory in order to evaluate rainwater quality and to analyze chemical stability of rainwater. The test results show that the collected rainfall water and surface runoff water were soft water and belonged to bicarbonate-calcium Type-I water category. The results also show that their pH levels were closely related to the strength of alkaline constituents in the rainwater. Water quality analysis indicated that the rainwater collected from the study region had been contaminated to a certain degree by nitrogen and organic compounds from intensive human activities. Water stability analysis focuses mainly on the potential of precipitation-dissolution of carbonate calcium (CaCO3). The results show that only the rainwater samples collected from traffic roads were oversaturated with respect to CaCO3 and had a tendency to precipitate, while other rainwater samples were unsaturated with respect to CaCO3 and had the potential to keep dissolving CaCO3 into rainwater. The results obtained in this study may help to develop better rainwater utilization strategies for alleviating the growing water shortage pressure faced by the City of Beijing.