The invention relates to an aquaculture net cage dirt collecting device, in particular a net cage dirt collecting device. The bottom of a net cage (2) is conical, and the net cage (2) is connected with a net cage dirt collecting hopper (1) in a conical shape; the net cage dirt collecting hopper (1) has a conical hopper body (1-1); an inclined plane (1-4) in the hopper body (1-1) is provided with an upward bent channel (1-3) which is upward connected with an upright cylindrical steam-water mixing pipe (1-2); the steam-water mixing pipe (1-2) has an enclosed cavity formed by an inner circular pipe and an outer circular pipe, wherein the peripheral wall of the inner pipe is provided with a plurality of small holes and the wall of the outer pipe is provided with one hole; an inlet pipe (1-5) is connected with an air compressor (8) through a compressed air pipe (3) and a ball valve (7); and the lower end of the inner pipe is connected with the channel (1-3), while the upper end is connected to a dirt collecting box (6) through a discharge pipe (4). Through the device, the dirt in the net cage is convenient to collect on the bottom of the hopper, and is sucked and lifted by a pipeline into the dirt collecting box. The device has the advantages of simple and convenient operation, and hard blockage in the pipelines. A mesh grid of an inner basket of the dirt collecting box can be madeinto various specifications, and the inner basket is light and handy and convenient to wash with water.