A locking mechanism (3) of a gauge-changing wheelset, a gauge-changing wheelset and a gauge-changing bogie. The locking mechanism includes: a traction electromagnet (32) and a locking pin (33), the traction electromagnet (32) includes a traction coil (32-1) and a movable core (32-2) being controlled to move by turning on and off the traction coil (32-1), the locking pin (33) is fixedly connected to an end of the movable core (32-2) away from the traction coil (32-1), a plurality of locking holes (41) are disposed on an inner hub of each of wheels (4) at intervals in an axial direction, and the movable core (32-2) may drive the locking pin (33) to insert into or remove from the locking holes (41) to lock or unlock the wheels (4). The gauge-changing wheelset includes wheels (4) and an axle (5) splined to the wheels, a locking mechanism (3) and lateral drive mechanisms (1), both ends of the axle (5) located outside the wheels (4) are respectively supported in an axle box body (7); the locking mechanism (1) is respectively disposed on the inner side of each of the wheels (4), the lateral drive mechanisms (1) are respectively disposed on the outer side of each of the wheels (4), and is fixed on the axle box body (7), the lateral drive mechanism (1) includes an electric cylinder (11) having an extension end for pushing or pulling unlocked wheels to change the gauge of wheels.