From the perspective of the causes for water quality accidents in urban tap water production-supply system, firstly, the risk factors for water quality are identified, and their coupling relationship is analyzed from single, double and multiple factors. Secondly, the risk coupling degrees of 362 major urban water quality accidents occurred in China from 2008 to 2017 are measured by means of the N-K model. The results show that the risk rate of water quality accidents increases with the increase of the coupling risk factors in the dynamic urban tap water production-supply system. The management factor is the internal cause for risks of water quality accidents, and the risk management is the key to avoid water quality risks. The environmental factors promote the occurrence of water quality accidents.