A grinding stroke control device for a valve stem grinding apparatus is fixed to a processing machine and provided for gripping a valve stem or a test bar. Using the hand wheel, the dial gauge and the positioning seat can obtain an initial position, then moving the measuring meter of the positioning mechanism along the transverse direction to make sure the valve stem has the right length with a standard valve clearance. By such arrangements, the grinding stroke control device for a stem valve grinding apparatus of the invention can have a less complicated structure and therefore is more convenient to operate.