It is very important to adopt a new index to evaluate me ratinality of energy consumption effectively, at the time to make great effort to reduce the energy consumption in water supply system. Current index of unit electricity consumption and integrated unit electricity consumption to assess the rationality of the energy consumption of the whole water supply system in our country cant be used to compare from landscape orientation. Aiming at the case, an index of efficiency is put forward on the base of analysis of the minimum energy that the water transportation in water supply system needs. According to the analysis of the index, the function to compare the energy consumption of different whole water supply systems of the index is proved.