An automatic mopping device with a driving worm arranged transversely includes an upper covering plate, an outer shell, a transmission member, an electrical component, a bottom cover, a chassis and a mopping ring assembly, and further includes a gear ring securing turntable, a mounting base and a motor cover. The overall height of the mopping device is between 60-64 mm, and the transmission member includes 2 wheels whose width is between 8-15 mm. The transmission member further comprises a perpendicularly arranged worm A and a horizontally arranged worm wheel B, a cylindrical spur gears A, B, C, a duplex gear and an internal gear ring. The duplex gear includes a cylindrical spur gear D and a planet gear. The worm wheel A and worm B are respectively meshed with the driving worm. The gear ring securing turntable is immovably connected to the internal gear ring, while the internal gear ring is meshed with the planet gear. The chassis is immovably connected to the gear ring securing turntable. The present invention has the following advantages: it is not that sensitive to the softness/hardness or the roughness/smoothness of the floor due to its relatively narrow wheels, thus avoiding breakdown, and its overall height is low enough to operate automatically underneath bed and table.