A child safety seat includes a child seat, and a support base having a shell body adapted to provide support for the child seat. The support base includes an anti-rebound part and a locking mechanism. The anti-rebound part is connected with the shell body for sliding movements along a sliding axis, the anti-rebound part having an abutting portion tilted relative to the sliding axis that extends upward at a front of the child seat, the anti-rebound part being slidable along the sliding axis relative to the shell body for displacing the abutting portion away from the child seat or toward the child seat so as to adjust a leg room for a child. The locking mechanism is operable to lock the anti-rebound part in position with respect to the shell body.