Most cities of our country now adopt the method of concentative pipe nets in water supply. In order to meet the users' request of hydraulic pressure, some small residential areas or building groups have established water supply systems of secondary compression. Now most water supply enterprises do much work in purification processes of waterworks so as to improve the water quality; however, the secondary pollution because of water supply pipe nets is ignored. An investigation was done by special team of "security technology of water quality in residential areas" and they investigated water quality of waterworks and health and quarantine stations in big and middle cities of the whole country. The result indicates that secondary pollution of pipeline networks indeed exists. So, in order to resolve the question of secondary pollution and to provide people with clean and secure drinking water, the reason that secondary pollution came into being must be understood, moreover, concrete methods and measures aimed at each reason should be found out.