Objective To simplify the structures of human upper extremity into a two rigid-body system by establishing a dynamic model based on a numerical equation approach. Methods Using Carerden angle as the generalized coordinates and applying the multi-rigid-body dynamic theory to establish dynamic equation; Using the MATHEMATICA calculation software to solve the equation numerically in the active and the converse solution, respectively and results were subsequently compared. Results The dynamic equation of the upper extremity with two rigid-body and numeric solution were obtained. The Carerden angle calculated by the known active force, which was calculated in the equation as the known motion, was accordance with that which was given when the motion was known. At one time the peak point appeared in the converse solution. Conclusion The MATHEMATICA software was a convenient approach, which was able to resolve the complex two rigid-body system equations in the upper extremity. In order to avoid the peak point, the evaluation should be conducted within a defined range with confined error range so that the solution obtained was acceptable.