The invention discloses a method for finely boring main shaft holes of an air cylinder body, which comprises the following steps of: finely boring the main shaft holes of a first step and a seventh step of the air cylinder body, and coarsely boring the main shaft holes of the second step to the sixth step; loading a boring rod by positioning the main shaft holes of the first step and the seventh step, and hanging a tool to clamp and fix the air cylinder; and connecting an upper horizontal borer to an upper main shaft to carry out fine boring of the main shaft holes of the air cylinder body. The invention also discloses a device for finely boring the main shaft holes of the air cylinder body, which comprises a main shaft hole fixture, the boring rod and a shaft cover component, wherein themain shaft hole fixture is connected with a main shaft clamp of the air cylinder body; the boring rod is arranged in the air cylinder body; and the shaft cover component is connected with the air cylinder body. The method and the device for finely boring the main shaft holes of the air cylinder body can ensure the coaxiality of the main shaft holes, reduce the machining cost and improve the accuracy and quality stability of products.