The invention discloses a device and method for grinding a tapered bore of a main shaft of a numerical control machine tool. The device for grinding the tapered bore of the main shaft of the numerical control machine tool comprises a bracket, a grinding awl and an awl sleeve fixing device. An installing hole is formed in the bracket. The bracket is fixed on the numerical control machine tool through the installing hole. The grinding awl is fixed on the bracket through the awl sleeve fixing device. The taper of the grinding awl is the same with the taper of the conical surface of the main shaft of the numerical control machine tool. The surface hardness of the grinding awl is greater than the surface hardness of the conical surface of the main shaft of the numerical control machine tool. The device for grinding the tapered bore of the main shaft of the numerical control machine tool can quickly and precisely grind the tapered bore of the main shaft of the numerical control machine on line, the machine tool fast restore the machining precision, a large amount of works such as main shaft disassembling are omitted, the downtime of numerical control equipment is shortened, a large amount of production cost is saved, and the labor intensity is relieved.