The value of the cable initial pre-stress has effect on the working performance of beam string structure(BSS).Theoretic and finite element methods were both used to analyze the inner force and deformation of BSS.It proved that finite element method which is accomplished by SAP2000 accorded with the theoretic one,thus SAP2000 were used to study the influence of initial pre-stress on mechanical performance.The result shows that rational value of the cable initial pre-stress can improve the internal force distribution of the structure,but do less influence on the vertical flexibility and support displacement.