Some researchers suggested that there was an ancient large lake covered about 15,000,000 square kilometers in Tibet, and when the water flew away and the lakes dried up, the crust would be deformed because of the unloading. Therefore, the angle of unconformity between lake sediments may provide new evidences for further research on the lake history. In this paper, we use a simplified mechanical model to calculate the lake-bottom deformation induced by unloading of reduction of lake water. We found that the tilt of lake bottom is negligible and difficult to be detected in the field. The maximum deflection is also quite limited, only about tens of meters. However, the uplift rate may be very fast, and its effects on environment, climate and physiognomy are worth to be further studied.