The invention provides a radial-flow air-flotation thickening apparatus for sludge. The apparatus comprises a contact area 1, a separation zone 2, a water outlet channel 3, a scum tank 4, a sludge tank 5, an air-dissolved water inlet pipe 6, a sludge inlet pipe 7, an effluent collecting pipe 8, a water outlet pipe 9, a slag discharge pipe 10, a sludge discharge pipe 11, a sludge and slag scraping machine 12 and a micro-nano bubble releaser 13, wherein the bottom of the sludge and slag scraping machine 12 is equipped with a sludge scraping plate 15 which is used for scraping sludge deposited at the bottom of a pool into the sludge tank 5, a slag scraping plate 14 is installed on a water surface and used for scraping thickened sludge into the scum tank 4, the micro-nano bubble releaser 13 introduces air-dissolved water into the bottom of the contact area 1, the effluent collecting pipe 8 introduces clear water at the bottom of the separation zone into the water outlet channel 3, the contact area 1 is located at the center of the separation zone 2, the water outlet channel 3 is located at the inner side of the separation zone 2, the scum tank 4 is hung on the sidewall of the water outlet channel 3, and the sludge tank 5 is located at the bottom of the separation zone 2. The apparatus has the advantages of a simple structure, easy maintenance, low cost, an ideal sludge thickening effect and applicability to large-scale sewage treatment plants.