To resolve the lane departure detection on structural road, monocular vision is adopted as the perception. Respectively based on the verification of projective geometry and the position of the vanishing point of lane lines in the picture with pinhole camera model, the relationship between lane departure and the ratio of lane line’s slope is concluded as a simple function which is unconcerned with the parameters of camera. Moreover, the experiment of measurement for the quantity of lane departure is performed with different camera orientation angles, and the result shows that the influence of small change of the orientation angles caused by the traffic direction can be neglected. Actual experiment result demonstrates that the precision of lane departure detection by this method is high, and the relative error is less than 5% compared with manual measurement. As stated above, the precision of lane departure detection by the method introduced is high and the complicated calibration of monocular camera can be avoided.