In the aim of reducing ineffective evaporation from reservoir waters in inland arid plain area, we conducted tests on plastic hollow plate covering the reservoir water surface. By preventing the water surface from external environment, the plates could suppress water surface evaporation, hence alleviating water shortage and improving the utilization rate of reservoirs. The wet area of plastic hollow plate consists of three parts: area wetted by wind waves, area wetted by the collision between plates, and area wetted by the collision between plates and dam slope. According to observed data we calculated the wet area in different meteorological conditions. And through statistical analysis on the probability of wind speed and corresponding evaporation reduction coefficient in a complete non-frozen period, we calculated the evaporation reduction amount and compared the water-saving efficiency in the presence of different cover area. According to results we obtained three different values of water saving efficiency: 55.38%, 65.97%, and 80.10% in three different test conditions. The results suggest when the plate area gets larger, the water-saving efficiency is higher. Moreover, the water-saving effect is related with test arrangement: when the plates are enclosed in a fence to avoid collision, the water-saving efficiency improves by 1.45% to the maximum; and when the plates are made integrate, the water-saving efficiency can be increased by 17.68%.