The invention provides a method for automatically estimating the posture of a vehicle-mounted camera, and the method comprises the following steps: obtaining two images which are taken by a camera in a vehicle in a certain time period; carrying out the extraction and matching of the feature points of the two images, and obtaining the vehicle real-time speeds obtained by two groups of matched points of the two frames; obtaining the real displacement conversion and rotation conversion in the time section of the two images according to the vehicle real-time speeds and the relative unit displacement conversion and rotation conversion of the two images; obtaining the matching information of corresponding points of AOIs of the two frames through an optical flow algorithm; obtaining a homography matrix of a road surface through RANSAC method; calculating the coordinates of a normal vector of the road surface in a camera coordinate system through a corresponding equation of the homography matrix, a rotation matrix and a displacement conversion matrix, thereby obtaining an external parameter matrix; and carrying out the automatic adjustment according to the external parameter matrix. The method avoids the troubles for a user in frequent calibration.