The invention relates to an automatic balancing mechanism for a platemaking roller. A corresponding device is arranged for the roller aiming at the unbalance generated on a platemaking roller body and different widths of plates on the platemaking roller body so as to realize the automatic balance under multiple sizes of plates. The automatic balancing mechanism is characterized in that: outer discs 14 at two ends of a roller 1 are provided with arc-shaped balancing grooves 17; a movable balancing block 18 is arranged in each balancing groove 17; and when different widths of printing plates are loaded, tail plate clamp bars 13 move to drive the balancing grooves 17 on the outer discs 14 to rotate so that the balancing blocks 18 move to corresponding balancing positions in the balancing grooves 17 along inner grooves of cams 19 and the whole roller is kept to be in a balancing state. The automatic balancing mechanism has the advantages that the same platemaking machine can be used for making multiplex widths of printing plates; a complex balancing drive control system is not required in the roller design, therefore the design cost of the roller and the production cost of the platemaking machine are greatly reduced; and the convenience for use and the easiness of general implementation are realized.