The invention belongs to the technical field of constructional engineering devices and relates to a stiffness structure and steel structure conversion device. A main body structure comprises stiffness structures, converting units, box-type steel structures, shear resistance studs, stiffness steel ribs and stiffening ribs; the stiffness structures and the box-type steel structures are fixedly connected with each other through the converting units, one sections of the stiffness steel ribs stretch out of the stiffness structures to enter into the box-type steel structures, the box-type steel structures are outwards coated on the H-shaped stiffness steel ribs and fixedly connected through the stiffening ribs, and the shear resistance studs are arranged on the inner walls of the box-type steel structures and steel web of the stiffness steel ribs. Concrete is poured at fixing positions of the converting units with sleeve connection structures according to construction modes of the stiffness structures so that fixed conversion from the stiffness structures to the box-type steel structures is achieved; and the main body structure is simple, reliable in design principles, simple in construction process, convenient to construct, good in reinforcing quality, high in effects and wide in application ranges.