Large complex mechanical products such as aircraft, rockets and submarines are composed of millions of different sized components. For example, in large-scale aircraft assembly, the length and width of large components are in the range of 10 ~ 100 m, but the boundary dimension of small components installed on the aircraft, such as inertial navigation system (INS) and head up display (HUD) system, is only in the range of 200-400 mm. The high-quality standard of the aerospace industry requires the position accuracy of small components to be kept within a tolerance of ±0.3 mm, which in turn places a stringent requirement on the measurement accuracy of the measurement system integrated on the aircraft assembly system. In this paper, the assembly accuracy of small comp onents with a planar surface in the digital assembly system is investigated since the planar surface is usually taken as the key product characteristic to evaluate the assembly accuracy of the small components.Digital assembly technology has been widely applied in aircraft assembly systems [1,2]. Naing studies the capabilities of jigless methodologies and principles with an automatic