We propose a method using azimuth markers to detect the optical-axis parallelism that affects the accurate measurement range of photoelectric theodolites. First, images of the azimuth markers were taken in the direct and reverse positions of the imaging system, respectively. Next, the three-dimensional coordinates of the cross hair corresponding to the range of the azimuth markers and the imaging angles of the cross hair were resolved. Finally, the optical-axis parallelism relative to the ideal collimating axis was detected using the formulas derived for optical-axis parallelism,enabling the detection accuracy to be analyzed. Errors associated with the coordinates of the projection center and the range of the azimuth markers influence the detection accuracy,which decreases as the distance to the azimuth markers increases. For anazimuth marker distance of 1 km, and error of coordinates of 1 cm, the optical-axis parallelism detection accuracy is 0.01 mrad. At sufficiently long distances, the accuracy of the optical-axis parallelismdetection has a considerable impact on the error associated with the imaging angle, thus satisfying the requirements of photoelectric theodolites. Thus, the problem of detecting the optical-axis parallelism affecting the measurement range of photoelectric theodolitesis resolved.