The serious liquid sloshing in the medicine tank of plant protection unmanned aerial vehicles (UAV) leads to poor flight stability. Aiming at solving this problem, a liquid sloshing restraining device, elastically constrained floating plate, was proposed by taking the weight reduction requirement of UAVs into consideration. A system of liquid sloshing force measurement was constructed based on a six degree of freedom motion simulation platform. A series of tests were conducted to investigate the sloshing suppression effect of the floating plate under various liquid depths and excitation frequencies. The effects of the structural and constraint properties of the floating plate on the sloshing suppression were analyzed. The results were as follows: the sloshing suppression ability was relatively weak when the floating plate was free. A good sloshing suppression ability can be got at a certain range of liquid depths for a rigid constrained floating plate. The range of liquid depths at which the sloshing suppression ability was excellent can be enlarged for an elastically constrained floating plate. When the floating plate was located near the free surface of the liquid and the elastic rope can be pulled by the plate, the liquid sloshing force was only 1/3~1/2 of the original even if the excitation frequency was near the natural frequency. However, the sloshing suppression ability basically disappeared when the floating plate located deeply below the free liquid surface. The location of floating plate was affected by the length of the elastic rope. The length needed to be selected according to the varies of the liquid depths. The larger the aera of the floating plate was, the better the sloshing suppression ability was. However, good sloshing suppression can also be obtained for a smaller aera under some conditions. On the other hand, the elastic constrained floating plate accelerated the decay of the free liquid surface and significantly improved the damping ratio of the oscillatory system. It showed a good practical value for plant protection UAVs whose attitude changed frequently and the liquid in the tank was prone to sloshing greatly.