In order to satisfy the on-line detection of multi-optical axis parallelism for modern integrated photoelectric systems, a method of large distance multi-optical axis parallelism detection based on optical aiming and inertial measurement is proposed. An accurate and movable reference coordinate system is established by an inertial sensor. The measured optical axes can be aimed and their vector coordinates in reference coordinate system can be measured by the measurement system. The angle between the measured optical axes can be calculated by their vector coordinates and then the parallelism of measured axes can be detected by their angle at the same time. A prototype of the measurement system is built and its measurement uncertainty is evaluated by the Monte Carlo method. The proposed method is experimentally verified through an actual test between laser rangefinder and infrared thermal imager. The test results show that the measurement error of the parallelism between sighting axis and laser emitting axis can reach 24.4″ and the measurement error of the parallelism between sighting axis and infrared axis can reach 27.0″. The proposed method expands measurement range and improves measurement efficiency by movable measurement. Therefore, it provides a method for large distance multi-optical axis parallelism on-line detection in the wild.