The utility model discloses a full-immersion biological rotating disc sewage treatment device with an adjustable liquid level. The full-immersion biological rotating disc sewage treatment device comprises a denitrification tank, a biological rotating disc mechanism and a lifting adjustment effluent weir mechanism, wherein the biological rotating disc mechanism is arranged in the denitrification tank, and the biological rotating disc mechanism is immersed in the denitrification tank and can rotate in the denitrification tank; the lifting adjustment effluent weir mechanism is arranged on the tank wall at the rear end of the denitrification tank, and is provided with a rectangular effluent weir opening capable of being adjusted at the upper and lower positions of the tank wall at the rear endof the denitrification tank. According to the device, a lifting adjustment effluent weir mechanism with a rectangular effluent weir opening capable of adjusting the vertical position is arranged on the tank wall of the rear end of a denitrification tank; the rectangular effluent weir crest is a water outlet of the denitrification tank, and the liquid level in the denitrification tank can be adjusted by adjusting the rectangular effluent weir crest, so that the depth of a disc of a biological rotating disc mechanism immersed in the denitrification tank is adjusted, and the effect of optimallyremoving TN, COD and TP in sewage is achieved.