Abstract: It is an important safety problem that how to deal with goaf in open-pit mining area. Based on the research project of comprehensive exploration and treatment of hidden geological factors in Yuanbaoshan open pit coal mine and the project of comprehensive investigation and treatment of goaf (cavity) in east open pit coal mine in Pingshuo,this paper makes theoretical analysis combined with the actual site conditions of east open pit coal mine by means of field measurement,and the methods of theoretical analysis,model construction and field measurement are adopted,We analyzed the shape evolution law and characteristics of balanced arch under the combined action of vertical geostress and horizontal tectonic stress to study and improve the traditional theory of balanced arch,and revise the calculation method of balanced arch height,and verifiy in the engineering practice. The results show that the horizontal stress should be taken into account in the collapse process of the layered roof. The balanced arch height decreases with the increase of the lateral pressure coefficient,and the safe thickness of the roof decreases accordingly through on-site observation of the occurrence and development of roof cracks in goaf,it is concluded that the roof stability calculated by the improved PU’s equilibrium arch is greater than that calculated by the traditional PU’s equilibrium arch,the calculation results of the improved przewalski equilibrium arch are in good agreement with the practice. In order to ensure the safe operation of large-scale equipment in the open-pit working face,and then determine the safety thickness of the roof of the underlying goaf to provide a theoretical reference,at the same time,it provides a basis for stability of goaf in open-pit mine,and the results will also provide a certain reference and theoretical guidance for efficient,economic and safe mining of open-pit mine.