Abstract: In order to study the full-range torsional performance of prestressed Steel-HSC (high strength concrete) composite box beams, the whole torsional process is divided into two stages: a pre-cracking stage and a post-cracking one. Based on the spatial truss model theory with a variable angle, a series of equations for the complete history analysis on its torsional performance were derived in the paper by using the equilibrium, deformation compatibility conditions and stress-strain relationships. A simplified method of torsiobal analysis on composite beams was given out. The corresponding VB programme was provided out, which have visual interface for input and output. The comparison between test and predicted results shows that the method proposed is rational and meets relevant requirements.