A pneumatic cylinder of pneumatic tool capable of avoiding failure of bearings, including: a cylinder body having an internal cylindrical chamber in which a rotor is disposed, a rotary shaft being fixedly connected with the rotor, whereby high pressure air can flow into the cylinder body to drive and rotate the rotor and the rotary shaft; two end caps respectively covering two ends of the cylinder body, two ends of the rotary shaft respectively extending out of the cylinder body through the through holes of the two end caps; two bearings respectively installed in the cavities of the two end caps and fitted on two ends of the rotary shaft; and two airtight rings located between the bottom walls of the cavities and the bearings to achieve an airtight effect between the bottom walls of the cavities and the bearings