Abstract: The whole instability of structure issues is becoming more and more outstanding because of the increase of the space steel structure span in the course of engineering applications. In addition, steel structures have defects in themselves like the connection, material fatigue, stress concentration and welding, which will cause many uncertain factors and directly affect the safety of a large-span steel structure. The analytic hierarchy process (AHP) and fuzzy theory are applied to the comprehensive assessment of long-span steel arch structure safety. According to the characteristic of a long-span steel arch structure, a fuzzy comprehensive evaluation model of a large-span steel structure is established by using the second fuzzy synthetic evaluation. By the theory of analytic hierarchy process, the weight values of every component and sub-items of a large-span steel structure with experts’ experiences and research results are determined. The judgment matrix of weight vectors of every item and component identification of every sub-item of a large-span steel structure is provided for guaranteeing the accuracy of evaluation results of the large-span steel structure. Finally, by the comparison of the results from using this method and experiments, the feasibility of this method is verified, which may provide references for similar projects.