Abstract: This paper studies the lateral supporting mechanism of the prestressed cable for a mega steel frame-cable bracing structure to provide a theoretical basis for determining cable's parameters. Based on the mechanism and by analyzing the prestress, a theoretical equations for cable's lateral stiffness is derived. After the model test and a comparison with the theoretical result and the numerical value, the lateral supporting mechanism of the prestressed cable is finally verified. With the prestressing force offering the axial stiffness and the stability for the cable, the prestressed cable can sustain tension and pressure and provide an elastic restraint support for the mega steel frame-cable bracing structure.