We investigated the problem of river's Minimum Ecological Water Demand (MEWD) in Chinese mountain area on the premise of protecting the river hydrobiology's habitat and scientific operation of the reservoir hydropower station project. This paper uses Wetted Perimeter Method to determine the MEWD of the downstream of Maojiahe Reservoir dam, and compares the results from Tennant Method with that from other two methods. The results show that Wetted Perimeter and Tennant methods are in good agreement and the former method has some rationality, and calculates the river's MEWD as 0.22 m3/s.