The invention provides a roof type air conditioner and an air quantity adjusting method thereof. The roof type air conditioner is used for supplying air for a plurality of rooms and comprises a roof unit (1), an air supply tube and an air returning tube, wherein the air supply tube and the air returning tube are connected to the roof unit (1); the air supply tube comprises a plurality of air supply branch tubes (2); the air returning tube comprises a plurality of air returning branch tubes (3); each room is connected with at least one air supply branch tube (2) and at least one air returning branch tube (3); and each air supply branch tube (2) is provided with an air quantity adjusting device which independently controls air quantity of the air supply branch tube (2). According to the roof type air conditioner provided by the invention, the problem that the air supply quantity of a single room cannot be adjusted by a roof type air conditioner in the prior art can be solved.