A child safety seat comprising: a seat shell for receiving a child, the seat shell having a backrest, a front end opposite to the backrest, and a seat portion extending between the backrest and the front end; an extension member disposed adjacent to the front end of the seat shell and slidably connected with the seat shell, the extension member being slidable relative to the seat shell to increase or reduce a distance between a forward end of the extension member and the front end of the seat shell; and an extension locking mechanism operable to lock the extension member with respect to the seat shell; characterized in that: the extension member has a transversal portion extending across a width of the seat portion, and two bent portions that are respectively connected with the transversal portion and rise above the transversal portion at a left and a right side of the seat portion; and an anti-rebound part (140, 240, 340, 440, 440', 540) is carried by the extension member, the child safety seat being installable on a vehicle seat in a rear facing position with the anti-rebound part (140, 240, 340, 440, 440', 540) extending at an angle above the extension member and adjacent to a seatback of the vehicle seat, the anti-rebound part (140, 240, 340, 440, 440', 540) being movable along with the extension member relative to the seat shell. a seat shell for receiving a child, the seat shell having a backrest, a front end opposite to the backrest, and a seat portion extending between the backrest and the front end; an extension member disposed adjacent to the front end of the seat shell and slidably connected with the seat shell, the extension member being slidable relative to the seat shell to increase or reduce a distance between a forward end of the extension member and the front end of the seat shell; and an extension locking mechanism operable to lock the extension member with respect to the seat shell; characterized in that: the extension member has a transversal portion extending across a width of the seat portion, and two bent portions that are respectively connected with the transversal portion and rise above the transversal portion at a left and a right side of the seat portion; and an anti-rebound part (140, 240, 340, 440, 440', 540) is carried by the extension member, the child safety seat being installable on a vehicle seat in a rear facing position with the anti-rebound part (140, 240, 340, 440, 440', 540) extending at an angle above the extension member and adjacent to a seatback of the vehicle seat, the anti-rebound part (140, 240, 340, 440, 440', 540) being movable along with the extension member relative to the seat shell. the extension member has a transversal portion extending across a width of the seat portion, and two bent portions that are respectively connected with the transversal portion and rise above the transversal portion at a left and a right side of the seat portion; and an anti-rebound part (140, 240, 340, 440, 440', 540) is carried by the extension member, the child safety seat being installable on a vehicle seat in a rear facing position with the anti-rebound part (140, 240, 340, 440, 440', 540) extending at an angle above the extension member and adjacent to a seatback of the vehicle seat, the anti-rebound part (140, 240, 340, 440, 440', 540) being movable along with the extension member relative to the seat shell.