A split mechanism for a coaxial photographing device, especially a split mechanism between a photographing unit and a light projection unit of a photographing device used for a full-time surveillance to facilitate an image adjustment or maintenance, uses primarily the photographing unit as a base of support, with the removable light projection unit being assembled coaxially at an image taking end. After the light projection unit is disassembled and changes a position, it can be linked at a constant direction temporarily and can keep operating by using a linking device which is split linearly, such that a calibration light wave needed for adjusting the images can be provided, and the light projection unit can be easily aligned with an axis to be assembled at position upon assembling.