The invention discloses a reservoir water quality monitoring device, which comprises a buoyancy tank, a detection tank, a water pumping pipe, a water pump, a water quality testing meter, a water draining pipe and water storage tanks, wherein the buoyancy tank is provided with a location device, the detection tank is arranged in the buoyancy tank, the detection tank is provided with a water inlet pipe and a water outlet pipe, a water outlet switch is arranged on the water outlet pipe, the water pumping pipe is fixed to the buoyancy tank, the upper end of the water pumping pipe is connected with the water inlet pipe, the lower end of the water pumping pipe penetrates out of the buoyancy tank, the water pump is arranged at the lower end of the water pumping pipe, the water quality testing meter is arranged in the detection tank, the water draining pipe is connected with the water outlet pipe, a plurality of branch pipe switches are arranged on the water draining pipe, all the branch pipe switches are connected with the water storage tanks through water draining branch pipes, and all the water storage tanks are lower than the detection tank. The reservoir water quality monitoring device disclosed by the invention can be used for accurately detecting reservoir water quality and timely storing a water body specimen of each period to facilitate redetection.