Abstract: To investigate the influence mechanism of axial force fluctuation on the seismic behaviors of steel box columns, the seismic behaviors of steel box columns under 5 patterns of eccentrically fluctuant axial force were numerically analyzed by finite element simulation. The influence rules of axial force fluctuation on their load-displacement hysteresis loops, skeleton curves, displacement ductility coefficients and energy dissipation capacities were discussed. The research results show that the loading frequency of a fluctuant axial force has an significant effect on the seismic behavior of a steel box column—when the loading frequency increases, the seismic behavior deteriorates rapidly. A large amplitude of the fluctuant axial force results in the severe degeneration of load-bearing capacity of a steel box column. When the width-to-thickness ratio of plates in steel box columns is small, the effects of axial force fluctuation on the seismic behavior become conspicuous. As a result, the adverse effects of axial force fluctuation on the seismic behaviors of the columns should be sufficiently considered when the seismic measures of steel box columns in a large span spatial structure are established.