Abstract: In order to increase the payload's space of electro-optical platform, inner frame dispositions need to be optimized reasonably. The photoelectric encoder is mounted next to the spindle and using the parallel shaft transmission, which can expand axial space of inner frame effectively. The advantages and disadvantages of the concentric shaft transmission and parallel shaft transmission were analyzed, and applicability and limitations of parallel shaft transmission such as single gear transmission, elimination clearance gear transmission and steel belt transmission were expounded. The influence of elimination clearance gear transmission and steel belt transmission to angle measuring precision of photoelectric encoder were analyzed through experiment. Experimental results show that the angle measurement error of elimination clearance gear transmissionis is greater than 3' in a limited angle range, and the angle measurement error of steel belt transmissionis is less than 1' within a limited angle range. It provides a method for increasing payload's space of electro-optical platform.