In hogging moment regions,the cracking resistance of the beams can be effectively increased on adding prestressing to the composite beams with external tendons,The rotation capacity of a composite beam prestressed with external tendon in hogging moment regions is governed by local buckling.With tools of abaqus,inelastic finite element buckling analysis of composite beams in negative bending was performed,and the FE results are found agree well with the test results.By varying cross-section parameters,27 groups of composite beams under negative bending with initial geometric imperfection are studied by means of the FE method.The slenderness of the studied beam is defined and the formula for calculating the rotation capacity is presented.The formulae are simple in form,in good agreement with the FE result and expected to be useful for designing the composite beams of class Ⅰ and Ⅱ.