The invention provides an external diameter grinder which is pressurized by a cylinder; the large guide roller seat (9) of the grinder is provided with a pressurization device which consists of a cylinder (1), a supporting plate (2), a flexible compressing device (3), a pneumatic element box (10), a pressure gauge (11) and a manual valve (12); three cylinders are fixed on the supporting plate which can move forwards and backwards; the supporting plate, the pneumatic element box, the pressure gauge and the manual valve are arranged on the large guide roller seat and control the cylinders by connection pipelines; the piston rod extending end of the cylinder is connected with the flexible compressing device, thus driving the flexible compressing device to move upwards and downwards; during the grinding process, after the distance of a large guide roller (8) and a small guide roller (7) is adjusted, bearing rings (6) are sequentially piled one by one; three oilstones (5) and a splint (4) are arranged on the bearing rings; according to the size of the external diameter and the requirement of coarse grinding and fine grinding, the pressure and the up-down stroke of the cylinder are adjusted; the pressure is middle so as to lead two fulcrums of the three flexible compressing devices to be supported at the middle positions of respective splint symmetrically, thus carrying out the grinding of the external diameter.