Groundwater is an important factor of slope stability, and ninety percent of slope failures are related to the influence of groundwater. In the past, free surface calculations and the prediction of water inflow were based on Darcy’s law. However, Darcy’s law for steady fluid flow is a special case of non-Darcy flow, and many types of non-Darcy flows occur in practical engineering applications. In this paper, based on the experimental results of laboratory water seepage tests, the seepage state of each soil layer in the open-pit slope of the Yanshan Iron Mine, China, were determined, and the seepage parameters were obtained. The seepage behaviour in the silt layer, fine sand layer, silty clay layer, and gravelly clay layer followed the traditional Darcy law, while the gravel layers showed clear nonlinear characteristics. The relation among the permeability, the porosity and the non-Darcy coefficient is investigated. A coupled mathematical model is established for two flow fields, on the basis of Darcy flow in the low-permeability layers and Forchheimer flow in the high-permeability layers. In addition, we considered the effect of the seepage in the slope on the transition from Darcy flow to Forchheimer flow. Then, a numerical simulation was conducted by using finite element software (LELAC 2.2). The results indicate that the free surface calculated by the Darcy-Forchheimer model is in good agreement with the in situ measurements; however, there is an evident deviation of the simulation results from the measured data when the Darcy model is used. Through a parameter sensitivity analysis of the gravel layers, we found that the height of the overflow point and the water inflow calculated by the Darcy-Forchheimer model are consistently less than those of the Darcy model, and the discrepancy between these two models increase as the permeability increases. The necessity of adopting the Darcy-Forchheimer model was explained.