This specification discloses a variable protection board that can change the width of the opening of a paper shredder. A sliding block is disposed on one side of the paper shredder opening to change the relative position between the sliding block and the opening. When the shredder is operating, the opening width thereof can be changed to a narrower default state, preventing the user from putting fingers into it and getting hurt. To back the paper out, the opening width of the shredder is changed to a wider default state for the user to take out the paper. A switch can be disposed along the stroke of the sliding block. When the sliding block goes to the state with a wider opening width for backing out paper, the shredder is powered off at the same time to shut down the running motor and thus protect the safety of the user.