The invention relates to a sludge steel slag ultra light ceramsite and a method for preparing the same. The ultra light ceramsite comprises the following raw material components by weight portion: 10 to 35 portions of dewatered sludge, 10 to 25 portions of revolving furnace steel slag, and 30 to 80 portions of clay. The method comprises the following steps that: the dewatered sludge, revolving furnace steel slag and clay which are dried and crushed are weighed up according to the proportion and mixed with water; and the mixture is stirred to prepare a raw meal ball, and the raw meal ball is dried at the normal temperature and then sintered in a resistance furnace. The method and the ceramsite have the advantages that: the process is simple; the prepared ceramsite is a spherical solid withdeep brown or black brown rough surface; the bulk density is 400 to 500 kg/m<3>, and the grain density is 760 to 925 kg/m<3>; the ceramsite can float in water; the water absorption rate is 4.0 to 6.0percent, and the expansion rate is 17 to 30 percent; moreover, the ceramsite also has the characteristics of light weight, high strength, small water absorption rate, big specific surface area, and the like.