To solve the calibration problem of camera in lane departure detection, the imaging model is analyzed and the installing position of camera can be adjusted according to the slope and vanishing point of three or more lane lines in the image. At the same time, external parameters of the camera are directly set without complicated calibration. Based on this, 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. Therefore, the quantity of lane departure can be calculated so far as this ratio is measured. 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.