<p>Tortuosity is an important parameter to characterize the fluid flow properties between planar seals in the field of seal leakage mechanism and leakage rate prediction. In this study, a numerical simulation method for tortuosity during the loading process of sealing interfaces is proposed. Based on the actual point cloud data of two rough surfaces, a three-dimensional percolation grid model is established to simulate the evolution process of the sealing surface. By integrating the process with finite element method (FEM), the variations in porosity and tortuosity of the seal interface can be obtained. The polished and sandblasted surfaces were measured experimentally and analyzed as sealing interfaces. When the porosity exceeds 0.05, the tortuosity and porosity of nine sets of random surfaces exhibit an exponential correlation. With the new method, tortuosity can be calculated directly based on the measured surface. The present work may be helpful to understand the transport mechanisms of sealing surface and provide theoretical support to leakage calculations between planar seals.</p>