Abstract: The determination of cable's parameters is one of key problems in mega steel frame and cable hanging structure. The equations for the internal force and deformation under vertical load are deduced based on a simplified model, then according to Newton's law, the inffiuence of the gliding cable on the mechanical property and the structural deformation under horizontal load is analyzed, and the maximum internal force increment of the cables is obtained. At last, according to the rule that the deffiection of a mega beam in a regular service meets the requirement of codes, the method of first determining the sectional area then the initial pre-stress is presented. This theory is useful for preliminary designs and the performance judgement for this new structure.