The invention relates to a fish-grazing grass-controlling type river crab aquaculture pond water quality control device and method. In order to fill the blank in the prior art, the water quality control device comprises a floating raft and a net cage, the net cage is fixed to the floating raft, the floating raft is rectangular and comprises a rectangular grid plate frame, a handrail and at least four floating boxes, the handrail comprises a fence and stand columns, the net cage comprises side nets on the periphery, a bottom frame and a bottom net, three or four twisting ropes are evenly arranged on the bottom frame, twisting drums are arranged on the fence or the stand columns, the upper ends of the twisting ropes are fixed to and wound on the twisting drums, the bottom frame and the bottom net can be lifted or released by rotating the twisting drums, grass carps with the weight of more than 250 g are bred in the net cage in a stocking mode, meshes of the side nets and the bottom net are prismatic meshes, the side length of the prismatic meshes is 1.5-2.0 cm, and bamboo spars are arranged. The water quality control device is simple in structure and reliable in operation, submerged plants are fully utilized, the water quality of the matched pond is fresh, dissolved oxygen is rich, aquatic plants are flourishing, and an excellent growth environment is provided for the river crabs.