Abstract: Aiming at the extrinsic camera calibration with respect to a reference object without a direct view, an annular mirror based automatic calibration method is proposed. The camera captures the images of the reference object through the reflection of a known-radius annular mirror. The equations of the inner and outer circles of the mirror in the image plane can be obtained with ellipse detection method. The equation of the outer circle is used to get the position of the mirror surface in the coordinate frame of the camera, while the inner circle is used to optimize the position parameter. The rotation and translation matrices between the virtual image of the camera and the reference object can be obtained after solving the PnP (perspective-n-point) problem as the intrinsic parameter of the camera is already known. Finally, the real extrinsic parameters between the camera and the reference object are obtained according to the mirror imaging principle. Simulation and real experiment results show that the method is simple and automatic with high accuracy.