A calibration apparatus and method for optical system assembly is provided, applicable to a finite conjugate optical system to determine the optimal image-forming positions of the light source and the focus object lens of the finite conjugate optical system. The apparatus includes an external light source, a low magnification image-forming optical system, an electrical control system and a monitor. When the parallel beam generated by the external light source is parallel to the optical axis of the finite conjugate optical system, the low magnification image-forming optical system is used to magnify the two focal spots formed by the external light source and the internal light source of the finite conjugate optical system to be calibrated. Finally, by adjusting the related position of the focus object lens or the internal light source of the finite conjugate optical system, the optimal relative positions between the light source and the focus object lens of the finite conjugate optical system can be found. The calibration apparatus of the present invention has the advantages of simple structure, easy assemble and setup, high precision assembly and low cost, as well as the capability to perform optimal calibration of the relative position of the focus object lens and the light source of each individual finite conjugate optical system.