The invention discloses a rotary-drum type fly-shearing machine and a method for adjusting gear-pair return clearance. The rotary-drum type fly-shearing machine is shearing equipment used for shearing the head, tail, low-temperature and irregular parts of strip stock after rough rolling. The fly-shearing machine comprises a motor, a speed reducer, a transmission shaft, a fly-shearing machine body, a lower rotary drum, an upper rotary drum, a head-cutting upper shearing blade, a tail-cutting upper shearing blade, a tail-cutting lower shearing blade and a head-cutting lower shearing blade, wherein the motor is connected with the lower rotary drum through the speed reducer and the transmission shaft; a gear I, a gear II, a gear III and a gear IV are pressed and arranged on cone shafts at two ends of the upper rotary drum and the lower rotary drum through hydraulic oil respectively; and the upper rotary drum and the lower rotary drum are symmetrically provided with a pair of shearing blades respectively. A transmission side is in mesh transmission with a synchronous gear pair when the fly-shearing machine is in a started state, and an operation side is in mesh transmission with the synchronous gear pair when the fly-shearing machine is in a braked state, so that the prior master-minor gear structure and shearing-blade adjustment device which are complex in structure and difficult to adjust are cancelled, and the structure is simple. In addition, the gear-pair return clearance can be adjusted and eliminated through the repeated assembly of gear pairs and the rotary drums, so as to improve the utilization rate of equipment.