Abstract: This research investigated the elastoplastic fretting contact behavior of the rough surfaces at the piston crown-skirt joint in marine diesel engines. A coupled Boundary Element Method-Finite Element Method (BEM-FEM) model, which integrated Fast Fourier Transform (FFT) and Conjugate Gradient Method (CGM), had been developed to optimize piston design and enhancing reliability under actual operating conditions. Unlike traditional analyses of smooth surfaces, this study accounted for actual surface roughness, thereby providing a more accurate representation of contact status. The BEM-FEM model effectively combined the computational efficiency of boundary elements with the detailed modeling of material behavior afforded by finite element methods. The study simulated the working forces acting on the piston assembly, including combustion pressure, inertia, and lateral forces, to analyze the interactions between the piston crown and skirt. An entire engine cycle was simulated to elucidate the evolution of contact states under realistic loads. At the top dead center (0°crank angle), the maximum axial load resulted in elevated contact pressure and minimal slip. In contrast, at a crank angle of 315°, lateral forces became predominant, leading to increased shear stress and the slip regions. During the downward stroke, as combustion pressure diminished and inertia forced prevail, contact pressure decreased, thereby reducing the overall contact area. This phase was particularly susceptible to fretting damage due to the diminished normal force, which facilitated micro-slips between the contact surfaces. The increased variability in shear stress during this phase suggested a potential for slip and the initiation of fretting damage. These findings underscored the necessity of optimizing preload force and taper angle to enhance stability and mitigate wear. The study also incorporated varying preload forces and taper angles to assess their impacts on contact pressure, tangential shear stress, and stick-slip behavior. The results indicated that reducing the taper angle from 0.25° to 0° improved the distribution of contact pressure, minimized slip zones, and increased stable contact areas. Additionally, an increase in preload force correlated with a reduction in slip, with near-complete elimination observed at a preload of 2 000 N. These findings suggested that optimizing taper angle and preload force could significantly enhance the performance of the piston crown-skirt joint, thereby reducing fretting wear and prolonging component lifespan.