The invention discloses a connecting shaft lubricating device, which is characterized by comprising an outer ring and an inner ring sleeved in the outer ring through a bearing, wherein sealing devices are arranged at two combined ends of the inner ring and the outer ring; a sealing cavity is formed between the two sealing devices; the outer ring is provided with an oil inlet communicated with a sealing space, and the inner ring is provided with an oil outlet; and one end of the oil outlet is communicated with the sealing space, the other end of the oil outlet is arranged at the end of the inner ring, and the oil outlet is connected with a lubricating point through an oil delivery pipe. Compared with the prior art, the device has the advantages that: a rotating connecting shaft can be lubricated, continuous oil supply is ensured, sufficient oil is ensured in a cross joint or a sliding key of the connecting shaft, sufficient oil in the lubricating point is kept under the environment of much water, the water difficultly enters the lubricating point, and the lubricating effect is good.