The child safety seat disclosed in the present invention includes a seatback, a headrest and a belt. The headrest is disposed on the seatback, and is adapted to move relative to the seatback up and down. The headrest includes an inclined guiding portion disposed on a bottom portion of the headrest, and an indentation of the inclined guiding portion points upward. The belt is disposed between the seatback and the headrest. The belt slidably contacts the headrest when a passenger sits the child safety seat and is constrained by the belt, and the headrest moves downwardly so that the inclined guiding portion drives the belt to be close to a center of the headrest. A height of the headrest is adjustable, a shoulder belt of the belt automatically moves when the height of the headrest is adjusted, and the neck of the passenger can be constrained by the shoulder belt.