In order to study the vibration characteristics of reinforced concrete (RC) structures during fire and to develop the method of damage assessment after fire, 4 simply-supported RC beams (L1~L4) were designed. Firstly, the finite element model of the simply-supported beams (L1~L4) were calibrated by picking up the modal information before exposure to fire. Then fire tests were conducted on L1~L4 for 60min, 90min, 120min and 150min respectively. During the fires, the structural modal information were collected, and the attenuation formula of frequency was obtained by fitting the test results. In order to study the residual stiffness and bearing capacity of L1~L4 after the fire, the dynamic tests and bearing capacity tests were carried out. Finally, based on the modal information of specimens after the fire, by taking equivalent explosion time as the damage index, the support vector machine intelligent algorithm was used to predict the damage degree of the RC beams. Based on the Standard for Appraisal of Building Structure After Exposure to Fire (CECS252-2009), a comprehensive grading of damage index for simply-supported RC beams exposure to fire was established, and the damage index grading of L1~L4 were evaluated at the end.