The present disclosure discloses a child safety seat, which is for being installed on a vehicle seat. The child safety seat includes a base having an anchor part connected to the vehicle seat; a seat coupled to the base; a top rod fixed to the rear of the base; a tether assembly including a top tether and a tether fixing portion that are connected with each other, the top tether being connected with the top rod and the tether fixing portion being connected with the vehicle seat; and an alarm device including a switch which can be triggered when the top tether is tightened. The present disclosure also discloses a tether assembly and a support structure. The child safety seat can conveniently prompt that it is not correctly fixed, and can be conveniently fixed and adjusted, thereby improving the safety and riding comfort of the child safety seat.