Abstract: The direction vector of vertical axis of focal plane reference frame and analytical expression in cosine of a vector included angle measured from normal line for catoptric plane of a suppositional horizon outgoing plane were derived based on the five degrees of freedom analytical expression for a image line projected on focal plane to track the targets on sea surface by the panoramic photoelectric detecting system with both azimuth and pitching axes under the condition of threeframe disturbances and panoramic rotating image. The angle included between the direction vector of vertical axis of focal plane reference frame and the normal line of a suppositional horizon outgoing plane, which is less than /2, was acquired by subtracting /2 from arc cosine. The value from included angle multiplying 0.5 is taken as the static momentum of pitching reflector, which is a kernel of pitching position tracking algorithm adapting to arbitrary rotating angle for image line. In combination with the azimuth compensating algorithm (nondissertated in this paper), the tracking algorithm can make the image points of offshore targets to approximate to the original point of the focal plane from the beginning to the end. Simulation results and engineering test validate the algorithm.