The invention discloses a device and d method for cleaning pool bottom sludge. The device comprises a support device, a stirring device, a collecting device and a concentrating device, wherein the support device is moveable through the buoyancy support of water; the stirring device is supported by the support device and stirs and conveys sludge at the bottom of a pool to the collecting device; the collecting device conveys the collected sludge to the concentrating device; and the concentrating device dehydrates the conveyed sludge. Due to the adoption of the device and the method, the problems of difficult excavation, poor excavation effect and incomplete clean are completely solved and a riverbed is not damaged. The sludge is conveyed after concentrating so as to form high-concentration convey, reduce water convey and improve convey efficiency.