The invention discloses a fermentation machine for an organic fertilizer. The machine comprises a feeding port, a storage bin, a discharging port, an inner drum, an airbag made from a non-elastic material and a stepping motor, wherein the inner drum is in coaxial rotating connection with the storage bin; a plurality of air vents are formed on an inner drum wall; one end of the inner drum is coaxially connected with an output shaft of the stepping motor; the other end of the inner drum is penetrated through the storage bin; a filtering plate for purifying air is arranged on an end part of the inner drum; a magnetic ring, the size of which is less than the diameter of the inner drum, is arranged on one side of the filtering plate close to the interior of the inner drum; the length of the airbag is consistent with the internal length of the inner drum; one end of the airbag close to the stepping motor is fixedly connected with the inner drum; a screw ring which is in threaded fit with the inner drum is arranged at the other end of the airbag; a magnetic circle which is sucked with the magnetic ring is arranged on the end part at one end of the airbag close to the screw ring; a spiral blade is arranged on one section of the exterior of the inner drum close to the feeding port; and a stirring blade is arranged on one section close to the discharging port. Compared with the prior art, the fermentation machine can circularly flow air therein so as to prevent the problem of poor fermentation effect caused by excessive sealing.