Before city emergencies happen, one of the most important tasks is to develop contingency plans, and reasonable analysis of space evacuation vulnerability is an important part of contingency plans. Based on the analysis of evacuation vulnerability of emergency area, this article develops a critical evacuation cluster model, and searches for the critical cluster of evacuation area by heuristic searching algorithm. At the end of the article, it generates the maps of space evacuation vulnerability in city emergency with the technology of GIS, which provide technical support for the making of contingency plans of city emergency.