：To study the vortex-induced force model of the steel box girder with projecting slabs, a cable-stayed bridge with a main span of 160 m was taken as the engineering background in this paper. First, a numerical simula? tion method with two-way fluid-structure interaction was used to analyze the whole process of vortex-induced vibra? tion of a steel box girder with projecting slabs. The vibration amplitudes of the main girder section were calculated under different wind speeds, and they were compared with those of wind tunnel tests. Then, the time-history of vortexinduced force of the main girder was extracted based on the validated numerical simulation results. A mathematical model of the vortex-induced force for the steel box girder with projecting slabs was established according to the re? sidual values between the reconstituted and target vortex-induced forces. Finally, the vortex-induced response of the main girder was inversely solved by using the fourth-order Runge-Kutta method to further verify the accuracy of the proposed vortex-induced force model, and on this basis, the steady amplitude expression of vortex-induced vibration of the main girder was deduced. The results show that the lock-in range and the amplitudes of the vortex-induced vi? bration for the main girder obtained from the numerical simulations agree well with those obtained from the wind tun? nel tests, and the maximum amplitude occurs at the oncoming wind speed of 7.2 m/s; in addition to the most impor? tant fundamental-frequency component, there are also double- and triple-frequency components when conducting the spectrum analysis on the time-history of vortex-induced force of the main girder, which implies that the vortexinduced force of the main girder exhibits certain nonlinear characteristics; according to the proposed expression for the steady amplitude of vortex-induced vibration of the main girder, it is found that the steady amplitude decreases with the increasing absolute value of parameter P12, but it increases with the increase of the parameter P10.