The invention relates to a lubricating device, in particular to the lubricating device for a dovetail sliding rail of a machine tool. The lubricating device for the dovetail sliding rail of the machine tool comprises a support frame body, a lubricating attaching board assembly and a distance adjusting assembly, the lubricating device is suitable for lubricating the dovetail sliding rail, the lubricating device can adapt to lubrication of the dovetail sliding rails with different sizes and the inclined side surfaces with different inclination angles on the different dovetail sliding rails, andthe adaptability is high. The support frame body comprises fixing blocks, transverse sliding grooves, transverse sliding groove rods, a vertical sliding groove, a vertical sliding groove rod, baffle head plates and a wheel shaft frame, the two fixing blocks are arranged, and the lower ends of the two fixing blocks are fixedly connected through the wheel shaft frame; and the vertical sliding grooverod is fixedly connected to the upper end of the fixing block at the left end, the vertical sliding groove is formed in the vertical sliding groove rod, the two transverse sliding groove rods are oppositely and fixedly connected to the surfaces on the inner sides of the two fixing blocks, the transverse sliding grooves are formed in the front ends of the two transverse sliding groove rods and therear ends of the two transverse sliding groove rods, and the front ends of the two transverse sliding groove rods and the rear ends of the two transverse sliding groove rods are both connected with the baffle head plates through screws.