The relative pose estimation in case of monocular cameras is generally based on point features. However, it would be difficult to obtain ideal results using this method in a scene, where the textures are not obvious and corners are small. Therefore, we combine the measurement data obtained from an inertial measurement unit based on the homography constraint associated with a planar scene and propose a monocular relative pose estimation algorithm by combining the point and line features. The images of the two consecutive frames can be initially rectified to downward view images using the angle information provided by the inertial measurement unit. Subsequently, the point-line features are detected and matched. Finally, the relative pose of the downward view images can be solved by transforming the relative pose of the original images according to the homography constraint associated with the combination of point and line features. The simulation and real experiments prove that the proposed algorithm is effective and that the relative pose for monocular cameras can be effectively measured.