A locking mechanism of a drawbar braking castor includes an abutting base, an actuating space, a pressing member, a transverse sliding seat, a round ear ring, an embedded locking frame, a first resetting assembly, a spacing base, a flexible inhaul cable, a variable driving member and a second resetting assembly. When the transverse sliding seat is pressed downwards by the extensible drawbar, the embedded locking frame is pulled to move via the flexible inhaul cable, such that the locating flange of the gear ring could be interlocked by the embedded portion. Hence, the locking mechanism of the drawbar braking castor could be adapted to various mobile drawbar carriers with a spacing between the castors and extensible drawbars, significantly improving applicability of the products.