Hydropower development has an inevitable impact on local environment, while promoting socio-economic development and improving the quality of people′s lives. The loss and gain analysis of ecological en-vironment, expressed by currency in a unified dimension, may provide a basis for engineering decision be-cause of its general evaluation on environment. An evaluation index system is established and the benefits and costs from hydropower development are analyzed by taking the environment affected by two scenarios of cascade hydropower development in the Naqu River for an example. Results show that the direct ecological values from two scenarios of cascade hydropower in the Naqu River are 1.05×10⁸ and 1.46×10⁸ yuan per year and the indirect values, 5.13×10⁷ and 5.04×10⁷ yuan per year, while the ecological environment costs are 9. 40×10⁷ and 9.73×10⁷ yuan per year, respectively. The ratio of the direct environmental benefit to the environmental cost from the two scenarios is 1.1∶1 and 1.5∶1, respectively, which illustrates that the benefits exceed the costs and the scenario two is superior to the scenario one. The valuation methodology of ecoenvironmental value in the hydropower development is not yet mature and currently, there is no unified evaluation standard. So there are some differences in the completeness and accuracy of the evaluation contents, as compared with the practice. The evaluation system can only provide a reference basis for the construction of hydropower development and environmental protection.