Based　on　the　test　results　of　water　quality　for　seventeen　times　rain　and　rainfall　in　southeast　region　of　Beijing　city,　it　was　conducted　for　a　comprehensive　water　quality　investigation　of　the　rain-water　and　rain-runoff　in　the　area,　including　characterization　of　their　chemical　properties　and　analysis　of　the　water　stability.　The　investigation　results　show　that　the　collected　rain-water　and　surface　runoff　water　are　soft　and　Type-I　water　within　the　category　of　bicarbonate　calcium.　The　results　also　show　that　the　pH　is　well　related　with　alkaline　particles　in　the　rain-water　and　runoff　water　within　this　area.　It　indicates　that　the　rain-water　and　runoff　water　within　this　area　have　been　contaminated　to　a　certain　degree　by　nitrogen-compounds　or　hydrocarbons　from　intensive　human　activities.　Water　stability　analysis　in　this　study　focuses　on　its　potential　of　precipitation　and　dissolution,　in　which　carbonate　calcium　(CaCO3)　plays　a　dominant　role.　The　results　show　that　only　runoff　water　on　traffic　roads　prefers　to　precipitate　while　rain-water　and　other　runoff　water　in　the　areas　are　characteristic　of　their　solubility.　Therefore,　the　problems　of　polluted　rain-water　and　engineering　installations　corrosion　should　be　especially　awared　of　during　the　rain-water　usability.