Abstract: The e cient dynamics algorithm for multibody system has always been a research hotspot. In recent years, many e cient dynamics algorithms made progresses in improving the computational e ciency, but most of them cannot provide an explicit expression of the system's dynamics or resolve the constraint forces. Based on the above issue, a constraint force algorithm (CFA) and its serialized application for the dynamics modelling of an arbitrary multibody system in tree-topology are studied in this paper. The constraint forces for the system can be directly obtained by solving for the system motion through CFA. The presented algorithm is an extension of the original CFA which is only applicable to a single-chained system. It is obtained by analysing the dynamics and kinematics of an arbitrary node in a treelike system. Serialization of the algorithm is developed to reduce the computational cost to a linear relationship with the number of degrees of freedoms (DOFs). The accuracy of the proposed method is validated through a numerical simulation of a space robot with multiple manipulators. The e ciency of the serialized CFA is verified by comparing the CPU time of di erent algorithms.