An external parameter calibration method for an image acquisition apparatus is disclosed. The method includes acquiring images from images acquired by the image acquisition apparatus. The images contain reference objects acquired by the image acquisition apparatus during the driving of the vehicle. The reference objects in the images are divided into a number of sections along a road direction in which the vehicle is located, and reference objects in each of the sections are fitted into straight lines. Pitch angles and yaw angles of the image acquisition apparatus are determined based on vanishing points of a straight line in each of the sections. The sequences of the determined pitch and yaw angles are filtered. Straight portions in the road from the filtered sequences of pitch and yaw angles are obtained. Data of the pitch angles and yaw angles corresponding to the straight portions are stored to a data stack.