Globally, freshwater is a limited resource for human development. Social and economic development must consider water resources for sustainable development. With China's rapid economic development, renewable energy has been pushed to the forefront of the development agenda, and hydroelectricity is the most important renewable energy.However, hydroelectricity also consumes a lot of water, hence, it is important to assess the impact of hydroelectricity on the environment and water resources, so many researchers have focused their attention on calculation of the water footprint. This is an indicator of the amount of water used to produce goods and services.It provides a reasonable and broad perspective for consumers and producers to help understand their relationship with freshwater systems. The water footprint concept help spolicymakers and stakeholders to enact more rational policies for water resources management. According to the available global literature, there are three main methods used to calculate the water footprint of hydroelectricity: Gross water consumption, Net water consumption and Water balance. The dominant calculation method uses the gross evaporation from freshwater reservoirs divided by the annual power production. In previous studies, the water consumption from a reservoir is often completely attributed to hydroelectricity, but this apparently overestimates the water footprint (WF) of hydroelectricity, especially for reservoirs with multiple functions. The multi-function reservoir provides many ecosystem services, such as water supply,irrigation, flood control,and fishery,so the traditional water footprint counting method will produce doubling counting. We used a new approach, the cost allocation method, to quantify the water footprint of hydroelectricity (WF_h) by separating it from the reservoir footprint using an allocation coefficient (η) based on the ratio of hydropower income to the total economic benefit of the reservoir. Such an approach was used to study data from 1988 to 2004 for the case area of the Miyun reservoir, a multi-function reservoir,and the largest reservoir in the North China plain.More than 60% of the economic benefit was from flood control and water supply, and less than 40% of the economic value was from irrigation, electricity and fishery. The results show that,from 1988 to 2004,the average product water footprint of hydroelectricity(PWF_h)in the Miyun reservoir was 897 m³/GJ,according to the traditional calculation method.However,based on the improved method, the average PWF_h was only 127 m³/GJ,just 14% of the traditional method.Initially, the primary purpose of the Miyun reservoir was for hydroelectric generation, but since 1998, the amount of electricity generated has decreased. Using the traditional calculation method, the PWF_h in 2004 was 4529.5 m³/GJ, the largest amount for all the years considered, yet, according to the improved cost allocation method, the PWF_h was 67.3 m³/GJ, the smallest among all the years considered. It is easy to obtain data using the cost allocation method and it has good operability.Application of the improved method will be propitious in the assessment of the market value of economic benefits. The allocation coefficient(η)combines the environmental indicator with the economic indicator, and the approach based on the allocation coefficient is more objective and accurate for the assessment of WF_h. The cost allocation method will further the application of the water footprint in hydroelectric power generation. Most reservoirs have a varying surface area over time, as a result of changes in water volume during the year and between years, thus, it is necessary to maintain temporal and spatial consistency when collecting economic data. Hydropower has long been considered a clean, renewable energy and has been actively promoted in many countries. This study indicates that the WF of hydropower should be integrated into the environmental impact assessment system for a more comprehensive understanding of hydropower projects in future. The cost allocation method can assess the influence of hydroelectricity in one reservoir andwill provide the basis for water price.It will also foster new ideas about how to accurately calculate the water footprint of hydroelectricity.