Abstract: In order to evaluate deformation capacity of corroded R. C. beams, factors affecting flexural stiffness of corroded R. C. beams are investigated, and a conclusion is reached that key factors affecting the establishment of calculation model are steel strain between cracks tending to uniformly distributed and steel strain lagging behind concrete strain for bond degrading due to steel corrosion. Throwing off conventional plane hypothesis, the section stiffness of flexural members with deteriorated bond strength is analyzed by means of stiffness analytic method. A comprehensive strain coefficient is adopted to reflect effects of both steel strain tending to uniformly distributed and lagging behind, and a calculation model of flexural stiffness of R. C. beams with corroded bars is established. By means of finite element analysis, the relationship between comprehensive strain coefficient and reduced coefficient of bond strength is built. The formula is used to analyze flexural stiffness of some tested members, and calculated results coincide with test results well.