The combination photo electronic sighting device usually composed of different optical devices, including telescope and low light level night vision device or infrared thermal imaging system. Every optical axle of the optical device of the combination sighting device is needed to be parallel. When a target away enough from the sighting device is observed, if the images of the target locate on the center of the field of the vision of the different optical devices, it shows that the axis are parallel. According to this principle, a rectifying device for optical axis of sighting device is designed. It includes off-axle paraboloid reflector, the target of cross line, light sources, precision modifying devices and other devices. The target of cross line is put on the focus of the off-axle paraboloid reflector, thus it forms a parallel light beam through the off-axle paraboloid reflector, the sighting device is immerged in the parallel light beam. The observing effect is like a target of long distance. The target is connected with a precision modifying device, by moving modifying device, until the image of the target coincide with the center of the field of vision of every optical device, whose degree of deviation of the optical axle can be calculated. The paper gives the structure of the rectifying device and the way of testing. The parameters of the device is given too.