In a mechanical extending device of a seat, a seat mounting plate limiting structure, a backrest hinge shaft, and a first connection rod hinge shaft are arranged on a seat mounting plate; a leg
assembly is mounted to a front of the seat mounting plate; a rear upper end of the seat mounting plate is hingedly connected to the backrest mounting rod. The backrest mounting rod, the backrest support rod, the first connection rod, the seat mounting plate, the third connection rod, the second connection rod, the leg
assembly are hingedly connected to each other. The second connection rod is arranged with a first support to support a load on the front of the seat mounting plate at a sitting position and a TV position. A second support is arranged on the third connection rod and is limited by a second support mounting groove of the fixing plate.