The invention discloses a low-speed transmission device of a mill. A motor coupled-type reduction gear is adopted to serve as a prime motor of the low-speed transmission device. The low-speed end of the reduction gear is provided with an overrun clutch; a star wheel of the overrun clutch is sheathed onto the low-speed end of the reduction gear; an outer ring of the overrun clutch is connected with a coupling half through a pin, wherein the coupling half is arranged on a high-speed shaft of a main reduction gear of the mill; and a main motor of the mill is connected with the other side of the high-speed shaft of the main reduction gear through the coupling half. The low-speed transmission device of the mill is simple in structure, high in reliability, small in size and convenient to use. When the main motor is started, the clutch does not need to be manually released, and the starting torque of the main motor of the mill can be greatly reduced.