The natural non-spherical incongruent hip joint cartilage surface is normally assumed as spherical in shape, which has been extensively applied in orthopedic clinic, hip joint simulation studies and hip joint prosthesis design. The aim of the study was to investigate the contributions of non-spherical incongruent hip joint cartilage surface to the hip joint contact stress, and to assess the effect of simplified spherical assumption on the predicted contact stress. Based on our previous anatomic studies that the acetabular cartilage surface was demonstrated as rotational ellipsoid in shape, three finite element (FE) models involving the natural hip joint cartilage shape, the hip joint cartilage shape replaced by the rotational ellipsoid and the sphere, respectively, were developed using the computed tomography (CT) image data of healthy volunteers. The FE predictions of contact stress on the replaced hip joint cartilage surface were compared with that on the natural hip joint cartilage surface. The result showed that the non-spherical hip joint cartilage surface contributed to the optimal contact stress magnitude and distribution. The replaced fitting spherical surface led to the increased contact stress of hip joint and the uneven distributed patterns of contact stress, whereas the replaced fitting rotational ellipsoid surface was comparatively more consistent with the natural results than the sphere one. The surface fitting error of the replaced rotational ellipsoid was fewer than that of the replaced sphere. These results indicate that the simplified spherical assumption will lead to misestimating the contact mechanics of hip joint, and the rotational ellipsoid model rather than the sphere model may represent the hip joint contact surface applied in the hip joint simulation study and the hip joint prosthesis design.