Abstract: To evaluate correctly the bearing capacity of a prestressed steel-HS concrete composite beam on experiment basis, the formulas to calculation the slip on steel/concrete interface in the composite beam are derived according to actual loading conditions, with the moment-curvature relationship, the change of interface strain and balance condition of internal force taken into account. The calculated results conform well to the experimental ones. It is indicated that the steel bar slip relative to concrete on steel/concrete interface is in a barrel-like distribution along the beam length, i.e., big at middle and small at both ends. In addition, the slip on interface decreases with increasing connection stiffness under otherwise equal conditions.