A barrel transmission shaft permanent magnet coupler capable of adjusting coupling space and area of an air gap magnetic field is of rotary barrel structure or rotary disc/rotary barrel composite structure and comprises at least a group of axial magnetic field armature winding permanent magnet coupling components or/and radial magnetic field armature winding permanent magnet coupling components, at least a pair of driving permanent magnet coupling rotary disc coupling mechanisms adaptive to driving rotary discs in the permanent magnet coupling components and corresponding driving shaft couplings, at least a pair of driven permanent magnet coupling rotary disc coupling mechanisms adaptive to driven permanent magnet coupling rotary discs in the permanent magnet coupling components and corresponding driven shaft couplings, a pair of permanent magnet coupling rotary disc air gap space and coupling area adjusting mechanisms and a pair of integral assembling mechanisms ensuring the system to be integral structure so as to be convenient for packaging, transporting and installing. The coupler is suitable for the technical fields of transmission shaft coupling drive and load speed regulating and the field of power dragging.