A worktable-elevating device for a planer is assembled on a worktable, including a base body provided with an input gear unit, two output gear units; a control rod, two elastic compressing units, and a worm holder. The input gear unit is driven by a motor installed on the worktable to rotate and actuate the two output gear units to rotate reversely. The control rod is pulled upward or downward to make the worm holder mesh with one of the two output gear units. As the worm holder and the planing table are screwed on the worm of the base body; therefore when the worm holder is driven to move upward or downward, the worktable will synchronously be moved up or down quickly. When the control rod is released and automatically recovers its original position by the elastic compressing units, the worktable will stop moving up and down.