The invention provides an extension transmission assembly of an automatic sofa iron frame. The sofa iron frame comprises a fixed part and an extension part, and the extension part is riveted on the fixed part in a foldable mode; and the extension transmission assembly comprises a first connecting piece, a push rod motor and a second connecting piece, and the first connecting piece is a groove-shaped connecting rod, and comprises a first fixed end and a first riveted end which are opposite. The extension transmission assembly is simple in structure, a currently commonly used fixing mode that amotor is fixed to the sofa iron frame is changed, the push rod motor is suspended instead, thus the first connecting piece and the second connecting piece serve as load-bearing parts, the burden of the push rod motor can be greatly relieved, the solid degree of the extension transmission assembly is increased, the stability of the extension transmission assembly is improved, and the service life of the extension transmission assembly is prolonged.