The invention discloses an overwater floating device for inhibiting algal bloom and reducing water evaporation. The overwater floating device comprises an isolation plate, a hollow floating ball and a connecting rod, the isolation plate is mounted at the upper end of the connecting rod, the floating ball is mounted at the lower end of the connecting rod, the isolation plate comprises an arc-shaped portion and a square horizontal portion, the horizontal portion is mounted at the bottom of the arc-shaped portion, the upper end of the connecting rod is positioned at the center of the bottom surface of the horizontal portion, the connecting rod is perpendicular to the horizontal portion, the side length of the horizontal portion is larger than the diameter of the floating ball, the bottom of the floating ball is horizontal, a balancing weight is arranged in the floating ball, and the connecting rod keeps vertical through the balancing weight. The overwater floating device is simple in structure, reasonable in design and low in cost, inhibits algal bloom in water, reduces environmental pollution and water evaporation and avoids loss of water resources in lakes, rivers, reservoirs and wet lands.