A braking system for a passenger boarding bridge that comprises a tunnel having a cabin, a cab and a rotationally driving device that comprises a driving motor mounted on the cab, a driving wheel mountable on an output shaft of the driving motor, and two driven wheels mountable on the cab and one drive belt being in cooperation with the driving wheel and the driven wheels, and having two ends fixable on the cabin respectively, the braking system comprising: the drive belt; the driving wheel; the driven wheels; a locking member configured to be fixedly mounted on the cabin; at least one set of braking assembly, each of which comprises an oscillating member configured to be rotatably mounted on the cab, an oscillating wheel mounted on the oscillating member and an elastic member, wherein the elastic member is configured to be pre-tightened by a pre-tightening force, the oscillating wheel cooperates with an outer side of the drive belt to form an abutting force so as to balance the pre-tightening force of the elastic member when the drive belt is in a normal working state, the pre-tightening force of the elastic member rotates the oscillating member to a direction of the locking member so that the oscillating member cooperates with the locking member in a locking manner to realize braking when the length of the drive belt between the driven wheels and the driving wheel exceeds a preset value or is in a broken state, wherein, the locking member is a chain or a rack, and a plurality of meshing teeth that can be engaged with the chain or the rack are provided on the oscillating member, and wherein, the oscillating member matches with the locking member in a locking manner to realize braking when the drive belt is loosened or the portion of the drive belt between the driven wheel and the driving wheel is broken. the drive belt; the driving wheel; the driven wheels; a locking member configured to be fixedly mounted on the cabin; at least one set of braking assembly, each of which comprises an oscillating member configured to be rotatably mounted on the cab, an oscillating wheel mounted on the oscillating member and an elastic member, wherein the elastic member is configured to be pre-tightened by a pre-tightening force, the oscillating wheel cooperates with an outer side of the drive belt to form an abutting force so as to balance the pre-tightening force of the elastic member when the drive belt is in a normal working state, the pre-tightening force of the elastic member rotates the oscillating member to a direction of the locking member so that the oscillating member cooperates with the locking member in a locking manner to realize braking when the length of the drive belt between the driven wheels and the driving wheel exceeds a preset value or is in a broken state, wherein, the locking member is a chain or a rack, and a plurality of meshing teeth that can be engaged with the chain or the rack are provided on the oscillating member, and wherein, the oscillating member matches with the locking member in a locking manner to realize braking when the drive belt is loosened or the portion of the drive belt between the driven wheel and the driving wheel is broken.