Abstract: 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 in- vestigation results show that the collected rain-water and surface runoff water are soft and Type-Ⅰwater within the category of bicarbonate calcium.The results also show that the pH is well related with alkaline par- ticles 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 a nitrogen-compounds or a 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 character- istic of their solubility.Therefore,the problems of polluted rain-water and engineering installations corrosion should be especially awared of during the rain-water usability.