Currently, the study of karst collapse column water inrush mechanism often ignores the effect of pore structure, and the traditional fractal seepage model ignores the effect of nonlinear seepage velocity field on model permeability. In order to solve this problem, the fractal seepage theory is combined with variable mass seepage theory, and the influence of seepage velocity field on model permeability is considered. The fractal seepage model of the third flow field of the settling column is established and improved, and the influence of different pore structure parameters and nonlinear seepage parameters on the macroscopic seepage of the settling column is analyzed.