The invention provides an automatic ring cutting device for a glass tube, which comprises a cylinder base, a cylinder, a guide rail base, guide rails, linear bearings, a bracket, a pinion gear, a rack wheel, a glass cutter, electric magnets I, electric magnets II, a cambered press plate, a stepping motor, a lower bracket, a bearing and the like, wherein two parallel polished rod guide rails are installed on a glass tube production line base through the guide rail base; two linear bearings are fixedly installed on the bracket and are in movable fit with the guide rails; the cylinder is installed on the glass tube production line base through the cylinder base; a piston of the cylinder is installed on the bracket; the stepping motor is installed on the bracket; the pinion gear is installed on the output shaft of the stepping motor; the rack wheel is installed on the bracket through the bearing and is meshed with the pinion gear; the electric magnets I are installed on the rack wheel; the glass cutter is installed on the shaft of the electric magnets I; the lower bracket is welded at one side of the bracket provided with the stepping motor; the two electric magnets II are installed on the lower bracket; and the cambered press plate is installed on the shafts of the electric magnets II. The invention can realize an automatic ring cutting function through a glass tube arranged at the center of the big gear of the device.