The invention discloses a worm wheel and worm device for automatically adjusting a transmission clearance. The ises a worm wheel arranged on a box body and a worm which is engaged with the worm wheel, wherein the worm consists of a rod-shaped worm and a casing assembly worm; the rod-shaped worm and the casing assembly worm are coaxially assembled, and the peripheries of the rod-shaped worm and the casing assembly worm are connected into a whole through a spline; a clearance for adjustment is reserved between the rod-shaped worm and the casing assembly worm; and the rod-shaped worm is axially fixed on the box body through a right-end support and a bearing pedestal, the casing assembly worm and a left-end support are assembled into a whole, a sleeve is arranged on the left side of the box body, the bearing pedestal is arranged in the sleeve, an oil chamber is formed between the sleeve and the bearing pedestal, an oil output interface is arranged at the position, which corresponds to the sleeve, on the left side of the box body, one end of the casing assembly worm is provided with an end cover, and a spring is arranged in a cavity formed by the end cover and the rod-shaped worm. By the structure of the device, the transmission clearance can be conveniently controlled by controlling the pressure of pressure oil, and the inconvenience caused by the assembly and disassembly of transmission mechanisms and the match grinding of adjusting washers is avoided; and the device is particularly suitable for conditions with complicated mechanisms and crowded maintenance space.