The invention provides a device for automatically cleaning a net cage underwater and relates to a device for controlling and removing fouling organisms on a buoyant raft cultivation cage on the sea. A bamboo rod penetrates through round holes in the centers of all net cage partition plates in sequence, the net cage is drawn straightly into the longest supporting state to be identical with a net in diameter basically, and all the net cage partition plates are located on the same axis. The net scraper is installed on the net cage in a sleeving mode, locating rings are installed on the upper and lower partition plates of the net cage so that the upper portion and the lower portion of the net scraper can not break away from the net cage, a sand bag is installed under the net cage so that the time for the net cage perpendicular to the sea surface can be prolonged, the net cage is connected with a raft body through a hanging rope, and the upper portion of the raft body and a floating ball are connected through a buoyancy rope to form the device. According to the device, the net scraper can float to the top of the net cage when no tide exists, when the tide is increased to a certain degree, the net scraper can move to the bottom of the net cage, in the process, waves can make the net scraper do high-frequency motion on the local part of the net cage, and the motion has impact force to make it difficult for the fouling organisms to adhere to and exist on the net cage.