A protective device for a castor includes a mounting ring, and a driving piece secured on the mounting ring. The mounting ring has an outer periphery formed with a breakable portion which is located beside the driving piece. The driving piece has a first end extending from the mounting ring and a second end formed with a pull tab. When the mounting ring is released by tearing the breakable portion, the mounting ring is directly driven by the driving piece to remove from each of the two rollers of the castor. Thus, the user only needs to apply a single action to pull the pull tab of the driving piece so as to remove the protective device from the castor completely so that the protective device can be removed from the castor easily and quickly.