In the hydro-fluctuation belt of reservoir, long-time inundation and dipping lead to vegetation dying, soil loss, rock slide and collapse, and landslide, as well as destroyed mountains stability. The hydro-fluctuation belt of reservoir should not only be regarded as the disturbed area, but also the destroyed area of soil and water conservation facilities, especially in South China where vegetation grows well. The destroyed area is the difference between the surface area when the water level is below its normal level and the surface area before the reservoir construction. The area can be calculated by the measurement arithmetic and the simple estimate arithmetic.