Abstract: The evaluation of vibration-mode uncoupling degree is a fundamental issue for vibration isolation design of multi-degree-of-freedom rigid-body-elastic-mounting system.Three conventional evaluation approaches,i.e.,the coupling-stiffness-based method,the vibration-mode-shape-vector-based method and the kinetic-energy-distribution-based method,are analyzed,and the deficiencies of conventional methods are pointed out.Using the concepts of the work which is done by the generalized inertia forces or by the spring rebound forces,the kinetic-energy-distribution-based method is modified to a modal-work-index-based method.The modified method is clearer in physical concept and is consistent in units for evaluation quantities,the modal-work is an appropriate measure to decouple the rigid mode and is applicable to vibration systems with different types of generalized coordinates,such as translational and rotational displacements.The modified method is an effective remedy and extension to the kinetic-energy-distribution-based method.Two examples are illustrated to exhibit the efficiency of the newly raised measure and its precision of the physical concepts.