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The long gallery type and cylinder type are the two main types of surge shaft in large scale hydropower underground complexes. The assessment method using FEM calculation results to analyze the structural stability of different types of surge shaft in large scale underground complexes is put forward. Firstly, the general FEM procedure to simulate underground excavation is proposed. Then, the FEM-based stability assessment method is expounded by introducing certain stability indexes. Such indexes are the distribution and volumes of plastic and crack zone, the magnitude of plastic dissipation energy and the distribution of deformation and stress around surrounding rock. A case study is further illustrated with the simulation of different types of surge shaft in a large scale hydropower underground complex. The comparison results shows that the proposed numerical method and corresponding stability assessment indexes are effective and reliable in evaluating different types of surge shaft. And it is concluded that the surrounding rock stability of cylinder typed surge shaft is more favorable than the long gallery typed surge shaft.