To obtain accurate finite element model （FEM） for health monitoring of concrete beams under fire exposure，a stepwise modification method based on the support vector machine （SVM） is proposed， which mainly considering the influence of boundary conditions and multi-parameters. Then， four rectangular beams and three T-beams are taken as the research objects，and the initial FEM is modified by using the first two order frequency and mode shapes measured. Finally，the fundamental frequency attenuation law of structures under fire exposure is studied based on model modification. The results show that the updated FEM and attenuation curve of fundamental frequency under fire exposure can better reflect the real dynamic characteristics of concrete beams. The proposed stepwise updating method can reduce the amount of computation，and can be effectively applied to the FEM of beams. Finally， to further study the influence of the variation of physical parameters on the fundamental frequency of concrete beams after fire exposure，the parameters such as section width， span-depth ratio，elastic modulus and fire exposure time are analyzed based on FEM. The attenuation formula of fundamental frequency with fire exposure time is fitted. The rationality of the formula is also verified by numerical and experimental results， which can provide reference for subsequent damage identification and assessment.