The utility model belongs to the technical field of gear machining and particularly relates to a grinding device for a gear key. The grinding device comprises a base, a column, a one-piece slide cylinder and a medium-load slider, the base is used for bearing a gear, the column is arranged on the base, the one-piece slide cylinder is arranged on the column, the medium-load slider is slidably arranged in the one-piece slide cylinder, and a grinding block used for grinding the gear key is arranged on the outer surface of the medium-load slider. During grinding, the gear is placed on the base, the column is adjusted in position, the grinding block is enabled to extend into the key, the one-piece slide cylinder is started to drive the medium-load slider and the grinding block to perform straight reciprocating motion, and two sides of the key are ground by the grinding block. Therefore, the grinding device has the advantages of high grinding speed and good consistency in grinding size.