Belt sludge dryer removes water of sludge on the conveyor belt in drying channel by circulating dry air so as to reduce the size and weight of sludge. The product can treat sludge from belt-type, centrifugal, and plate-and-frame dehydrators. Dehydrated sludge is distributed on ventilated conveyor belt which slowly moves forward, and then water of sludge is evaporated by continuously circulating dry air. The whole drying process is finished in a sealed environment from which smell and dust will not escape.