In order to increase the efficiency and reliability of the dynamic analysis for spatial multibody mechanical systems containing the coupling of multienergy domains,a method based on bond graph was introduced.From the viewpoint of energy conserving,the peculiar property of bond graph multiport element MTF was discussed.Based on this,the procedure of modeling spatial multibody mechanical systems by bond graphs and its dynamic principle were described.To overcome the algebraic difficulty brought by differential causality and nonlinear junction structure,the constraint forces at joints can be considered as unknown effort sources and added to the corresponding 0junctions of the system bond graph model.The unified formulae of system state space equations and constraint forces at joints were derived,which could be easily generated on a computer in a complete form.As a result,the automatic modeling and simulation on a computer were realized.The validity of the procedure was illustrated by a practical example.