The equations of damage evolution of cross-section and the deflection deformation of concrete beam were established. Through the plain concrete beam bending test, the formula for calculating the deflection was verified. It is proved that the formula established in this paper is more accurate than the equation that doesn’t consider the damage effect. The concrete beam was modelled and analyzed by means of finite element analysis software, and comparison was carried out between the numerical simulation result and the experimental data. On the basis, new methods and ideas about concrete-damage were proposed.