In the aim of restraining invalid evaporation of plain reservoir in arid area, the efficiency of PVC lightweight balls covering the water surface of reservoir in reducing surface evaporation was evaluated. The movement of lightweight balls in the presence of difference wind speeds was observed and analyzed through indoor and outdoor tests. The causes of irregular rotation and rolling of balls were investigated, and additional load weights were added to the balls to solve this problem. Furthermore, the wetting rates and anti-evaporation coefficients of balls with different load weights under different climate conditions were calculated and compared. Results suggest that floating balls with additional load weights have better effect of reducing evaporation than balls with no additional load weight. The comprehensive rate of reducing surface evaporation could improve by 24% to the maximum.