The present invention relates to the field of water sluice gate cleaning technology, in particular relates to an underwater cleaning device for a gate slot and a use method thereof. The underwater cleaning device for a gate slot comprises: a frame, wherein the frame comprises a bottom plate and vertical braces provided at four corners of the bottom plate, each vertical brace is provided with a climbing structure, and each of the climbing structures is provided with a climbing bracket, the climbing brackets are fixedly connected to the vertical braces by means of connection rods, and the climbing brackets are configured to be electromagnetically attracted to an inner wall of the gate slot; a horizontal sensor provided on the bottom plate; a controller in communicational connection with the horizontal sensor and the climbing brackets, wherein, in a state of underwater, the horizontal sensor is configured to sense the levelness of the bottom plate and transmit the result to the controller in real time, the controller is configured to control the climbing brackets to move up and down along the gate slot to maintain the bottom plate in a horizontal state. The present invention solves the problem that an automatic cleaning equipment may be affected by multiple factors such as water flow, etc. in a state of underwater, which would cause the automatic cleaning equipment to tilt during movement along the gate slot and adversely affect the sediment cleaning effect.