The invention discloses an afterheat recovery system and an afterheat recovery method of dry granulating liquid slag. The system comprises a granulating cabin, a moving bed and an afterheat recovery system, the inner surface of the granulating cabin is provided with a granulating cabin water cooling wall, the center of the top of the granulating cabin is provided with a slag pipe, the slag pipe extends into the granulating cabin, the lower part of the tail end of the slag pipe is provided with a centrifugal granulator, the centrifugal granulator is connected with a motor arranged at the lower part of the centrifugal granulator, the lateral side of the centrifugal granulator is provided with an air supply pipeline, an annular air opening is formed between the centrifugal granulator and the air supply pipeline, the lower part of the granulating cabin is provided with the moving bed, the inner surface of the moving bed is provided with a moving bed water cooling wall, the top of the moving bed is provided with an air outlet, the bottom of the moving bed is provided with a slag discharge device, an air inlet device is arranged above the slag discharge device, the afterheat recovery system comprises a primary dust remover, an afterheat boiler, a secondary dust remover, an exhaust fan and a chimney which are connected in sequence, and the primary dust remover is connected with the air outlet arranged at the top of the moving bed. The system and the method effectively solve the problems of heat recovery and furnace slag granulation of equipment such as a blast furnace.