Abstract:Rock-bolted crane girder is a widely used structure in underground powerhouse nowadays. In hydropower projects longitudinal crack between rock-bolted crane girder and surrounding rock masses can hardly be found. Aiming at the cracking phenomenon in an underground powerhouse,detailed analysis of the cracking mechanism is made based on geology,monitored data and construction process. It is concluded that the cracking initiates between late October and early November,2006;differential deformation caused by vein faults in the surrounding rock and excavation is the major cause leading to the cracking while the development of the differential deformation cannot be effectively controlled by support measures. So considering the discontinuous characteristics of rock masses,it is important to make detailed geological investigation and to forecast possible differential deformation on the sidewalls during construction period,so as to take effective support measures and assure the safety of the girder.