The invention provides an online cleaning machine for an automobile exhaust treatment system, and relates to the field of automobile maintenance. The online cleaning machine comprises a fixed frame, a liquid storage tank, a gas storage bag, a pipeline circulation booster pump and a pneumatic diaphragm pump, the liquid storage tank and the pneumatic diaphragm pump are mounted in the fixed frame, and a first liquid outlet pipeline is arranged between the liquid storage tank and the pneumatic diaphragm pump. A second liquid outlet pipeline is arranged between the pneumatic diaphragm pump and the exhaust treatment system, and a liquid return pipeline is arranged between the exhaust treatment system and the liquid storage tank. According to the circulating cleaning process, on one hand, the exhaust treatment system does not need to be dismantled for cleaning, operation is more convenient and faster, the loss of automobile parts is minimized, the labor operation cost is saved, on the other hand, cleaning liquid can be utilized most efficiently through circulating cleaning, waste of the cleaning liquid is avoided, meanwhile, the cleaning cost is saved for an automobile owner, the market prospect is considerable, and the economic value is high.