In this paper, the data monitored by the cable dynamometer installed on the top arch of the underground main building of Xiaolangdi Project during excavation are analyzed, and the frictional loss of the hole in the post-tensioned cable tension is calculated. The analysis of the two shows that the increase of anchor cable tension reflected in the monitoring data at this stage does not indicate that the rock mass of the roof arch of the powerhouse has subsidence deformation due to excavation, but is the result of the adjustment of the stress of the anchor cable after locking, but the excavation blasting of the powerhouse during this period has a transient effect on the change of anchor cable tension.