The invention relates to a sludge-sucking device, in particular to a robot which can fully automatically suck sludge. The sludge-sucking robot comprises a suction pipe coiling assembly, a sludge pumping assembly, a suction pipe and a suction pipe guide assembly, wherein the suction pipe guide assembly is the intelligent kernel of the sludge-sucking robot; and the suction pipe guide assembly can automatically realize forward, backward, left-turn and right-turn motions in a water tank, automatically detect the depth of sludge to be sucked, extend the suction pipe into the sludge, and give orders to the sludge pumping assembly and the suction pipe coiling assembly on the bank to control the working modes so as to automatically suck the sludge from the water treating tank to a specified sludge collecting tank. Compared with the prior art, the sludge-sucking robot can automatically and thoroughly clean underwater sludge in an unattended mode, thereby thoroughly changing the traditional sludge-sucking mode, emancipating people from exhausting labor, saving the high dredging cost, and being a scientific, energy-saving and environment-friendly invention in deed.