A child safety seat includes a seat base and a safety-belt clamping device disposed on the seat base, and is disposed to a car seat by a safety belt. The safety-belt clamping device includes a bottom seat, a rotatable clamping arm connected pivotally to an end of the bottom seat, and a retainer mounted to an opposite end of the bottom seat and retaining releasably a free end of the rotatable clamping arm thereon. The seat base has a mounting groove. The bottom seat is disposed fixedly on the seat base. The rotatable clamping arm and the retainer are disposed within the mounting groove. When the rotatable clamping arm is retained on the retainer, the safety belt is clamped between the rotatable clamping arm and two sidewalls defining the mounting groove.