Extension support assembly of car seat

An extension support assembly of car seat arranged in a pad of a car seat and including a fixed plate, a controlling plate, a claw piece, an actuating piece, a linkage and an extending plate is provided in the present disclosure. The fixed plate is arranged horizontally and fixed on the pad, and multiple positioning teeth are formed at an edge of the fixed plate. The controlling plate is slidably jointed with the fixed plate and stacked on the fixed plate, and a guide groove is defined on the control board. The claw piece is stacked on and pivoted with the control board, the claw piece is engaged with one of the positioning teeth, and a guiding pin is arranged on the claw piece. The actuating piece is stacked on the control board and inserted in the guide groove, and a chamfer for pushing the guiding pin is formed at an edge of the actuating piece. The linkage is pivoted on the control board and pivoted with the actuating piece. The extension board is fixed with the controlling plate and arranged in a stackin