This paper focuses on the study of the variation characteristics of ultimate bearing capacity of reinforced concrete beams under fire within different design parameters to provide guidance for fire resistance design of concrete beams. The transient thermal analysis model of reinforced concrete beam is established by ABAQUS finite element software to calculate the temperature field distribution under fire， and the bearing capacity of the beam is analyzed by sequential coupling analysis method. The calculation results show that the increase of the reinforcement ratio of the beam section and the thickness of the protective layer can improve the fire resistance performance of the beam， but the increase range is limited. Through the quantitative analysis of the design parameters of the bearing capacity of reinforced concrete beams under fire， it is concluded that the increase of the thickness of the protective layer of the beam can improve the fire resistance performance more effectively than the increase of the reinforcement ratio of the section， and the rationality of the value of the thickness interval of the structural protective layer in the fire protection design code is verified from the perspective of quantitative analysis.