The invention discloses a gum-protecting device which is an integrated shell structure. The inner surface of the shell is matched with the half of the longitudinal section of a tooth drill body. The shell of the gum-protecting device is devised into a machine needle base connector, a body connector, a gum-protecting part and a positioning part. The gum-protecting part of the gum-protecting deviceis matched with the machine needle on the tooth drill; the inner diameter of the gum-protecting part is larger than the diameter of the machine needle; and a gap is remained at the fixed socket part of the machine needle of the tooth drill. The front end of the gum-protecting part is provided with a positioning part which is extruded out of the length of the front end of the machine needle; the machine needle base connector of the um-protecting device is covered at the external surface of the machine needle base of the tooth drill; the body connector of the um-protecting device is covered at the external surface of a handle of the tooth drill; the machine needle base connector and the body connector are tightly matched with the tooth drill body, so that the gum-protecting device can be stably fixed at the external surface of the tooth drill. The gum-protecting device has the positioning function in the process of polishing tooth shoulder, and effectively separates the machine needle from the gum to protect the gum.