A reservoir dredging assembly includes a dredging tube and at least one dredging device adapted to be installed to at least one secondary reservoir. The at least one dredging device includes a linking tube, an extending tube, a water gate, and a control unit. The linking tube is connected to the dredging tube for water in a primary reservoir, together with sediments deposited therein, to flow through. The extending tube has a top cutlet adapted to be located lower than the maximum water level of the primary reservoir. The water gate is mounted in the linking tube at a position downstream of the extending tube. The control unit is convertible between a first state for closing the water gate and a second state for opening the water gate.