It is difficult to characterize the whole process of seepage movement by traditional numerical methods, which leads to the unclear mechanism of stress-strain change in soil. Therefore, the meshless method smooth particle hydrodynamics method is proposed to study the seepage law and deformation characteristics of soil. Combined with the strain softening model, a nonlinear cohesive force change model of porous media is proposed to accurately describe the evolution characteristics of soil seepage failure. Secondly, the damping coefficient and stress normalization method are used to better eliminate the defects of numerical stress noise generated by soil deformation after boundary interpolation, and improve the accuracy of SPH method in studying the internal stress of soil deformation. It is concluded that the influence of the viscosity coefficient on the unsteady head is significantly higher than that of the constant head. At the same time, the evolution characteristics of soil seepage instability under linear cohesion change are compared and analyzed. It can be seen that the proposed nonlinear cohesion change model is more in line with the actual failure mode of soil seepage. The research results can provide a new method and idea for the study of slope seepage motion range and stability analysis.