Lake water reserves are often estimated according to water level observation and manually-measured underwater topography data. As for the lakes which lack data, it is really difficult to obtain the information of lake water reserves. In order to explore the solution of this problem, the authors chose the Nam Co Lake in Tibet as a study case. Based on the features of topography similarities between the above lake level and the under lake level, the authors made use of SRTM DEM of above the lake level to construct the relationship between the elevation and the area, the area and the volume increment. In turn, the elevation-area-volume increment of the lake's underwater was recalculated. Finally, the authors constructed the area-volume model to calculate the lake water reserves. The result shows that the calculation is of high relative accuracy. According to the area of the lake by using the DEM of the Nam Co Lake basin,the authors calculated the lake water reserves, which reach 111.570 billion m ³. This result is compared with the calculated water reserves of 101.950 billion m ³ in the Nam Co Lake underwater terrain digital elevation model established based on measured water depth data, and its absolute error is 9.620 billion m ³ whereas its relative error is 9.40%. The results obtained by the authors provide a reference for the estimation of natural lake water reserves with consistent water and underwater topographic parameters in data-absent areas.