Abstract: The influence of the position and quantity of fire span on the fire behavior of concrete floor in the structure is studied. Based on Vulcan software, using thick plate element and beam-column element, combined with EC2 material high temperature constitutive model, the temperature and deformation of fire top floor (corner and middle) and second floor (four) in a steel frame structure are simulated numerically. The influence of concrete expansion strain on the deformation of floor slab under fire is analyzed, and the membrane mechanism, crack distribution and internal force development of steel beam are investigated. The results show that: the predicted results of floor temperature and deformation are in a good agreement with the experimental results, and the concrete expansion strain has a decisive influence on the mid-span deformation of the fire zone in the structure, that is, when the expansion strain is larger, the predicted deformation is larger; the boundary conditions have a decisive influence on the membrane mechanism and crack distribution of the heated plate, and the heated plates have three kinds of membrane mechanisms: ring type, opposite-edge U type and diagonal U type. The bending moment and axial force of the steel beam near the heated grid gradually increase, and the increase of the axial force of the steel beam is relatively large.