In order to discuss the progressive collapse of long-span space structures with the alternate path method (AP method), a practical method for the evaluation of elements to be removed was presented. The elementary scope of important elements was selected through concept analysis, and then the importance coefficient of these elements was calculated through simplified sensitivity analysis to choose the important elements. The effects of the removal of elements, the variation of cross-section of elements and the change of pre-stress of cables on the structural response were considered in the sensitivity analysis. A space grid structure and a cable-arch structure were analyzed in this method. The results have shown that the method is suitable for the evaluation of important structural components of long-span space structures. The problem that the elements to be removed for other non-frame structures are not given in codes has been solved, and then the AP method will have extensive applications.