Abstract: Block theory is an effective and reliable engineering analysis method for the stability of fissured rock mass. Based on stereographic projection,an analysis is made for the stability of random blocks in surrounding rock of a certain circular surge shaft of a hydropower station. During the study with stereographic projection cylindric free surface is simply converted to eight walls. Potential key blocks and their moving modes are studied and the values of sliding force of those blocks are worked out. This could be a direction to the design of excavating,anchoring and permanent lining of those walls,and also a reference to similar projects.