Abstract: Analysing and evaluating the calculating mothod of water supply efficiency in cities which are recommended in 《Economic Evaluation Standards on Water Conservancy Development Projects》,the article points out the characteristics of all kinds of methods.It also uses the economic implication of the shadow price to make the preliminary research on the shadow water price method which is put forward but not definitely calculated in the paper above and applies it to the calculation of the water supply efficiency in cities on the Western Line diverting water from South to North of China.