In order to determine the physical parameters of upper-limb exoskeleton mechanism and evaluate its motion performance, a precise modeling method is proposed to deduce the motion space of human upper-limb. Depending on the physiological characteristics of human upper limb, the self-motion manifold is optimized and a method to avoid the singularity of upper limb exoskeleton mechanism is introduced. Firstly, the anatomical structure of upper limb is analyzed. The equivalent kinematic model including the angles of the sternoclavicular joint, glenohumeral joint, elbow joint and etc, is built to calculate the orientation of endpoint. The reachable space of upper limb motion is simulated accurately. Secondly, the optimal orientation of motion manifold is calculated according to the self-motion characteristics from the redundant freedom of upper limb while the maximum loading capability of upper limb is considered. Finally, the singularity orientation of upper limb is analyzed based on the proposed model. The configuration of exoskeleton system is given to avoid the singularity in exoskeleton system motion control through mechanism design.