The invention relates to a method for machining a central oil hole of a main journal of a crank of a medium-speed engine. The method comprises the following steps of: drilling the central oil hole of the main journal before the surface of the crank is quenched, boring the central oil hole of the main journal, drilling the central oil hole of the main journal through a drill jig tool, fixing a reference V-shaped block and a clamping V-shaped block of the drill jig tool and bolts on the main journal, requiring that the center of the drill jig and the center of the main journal are overlapped, arranging a parabolic bit on a drill stem, drilling through the oil hole of the main journal under the guide of a drill jig, and boring the central oil hole of the main journal on a deep-hole drilling-boring machine HZZ1, wherein the boring head is a general boring head and is in threaded connection with a connecting rod; the connecting rod is clamped on a boring rod of the machine tool; and when the boring head enters a certain main journal required to be bored, the connecting rod plays a role in supporting another main journal, and the boring of central through hole of the main journal is divided into heavy boring, fine boring and root R boring. By the method, the product quality can be guaranteed, and the production efficiency can be greatly improved.