Abstract: An improved method based on screw theory for constraint and mobility analysis of complex coupled multi-loop mechanisms is proposed. The innovation is the proposition of motion shunting measure,by which the complex multi-loop coupled mechanisms can be split into several independent elements. The relative motion of the coupled-nod link in each element was analyzed and replaced by a generalized kinematic chain. The coupled mechanism was ultimately regarded as a common parallel mechanism. Limb twist systems and limb constraint systems of the equivalent parallel mechanism were analyzed and the characteristic of the output mobility was identified. With the proposed method,a typical Hoberman Switch-Pitch ball was analyzed,which shows the validity and simplification of the new method.