In this paper, we try to find an efficient and accurate dynamic model for a robot with spatial compliant links using the screw theory. After making some reasonable assumptions for the system, we introduce the Ding-Holzer method for system simplification. Instead of looking at the beam at rest, we consider the spatial forces and deflections of a moving elastic arm. We focus our study on the dynamic modelling problems of a compliant arm with six dimensional forces and deformations at the tip.