Abstract:The deformation characteristic of surrounding rock in the underground powerhouse of a hydropower station is analyzed by R/S method and fractal theory. Hurst indexes and fractal dimensions of displacement time series of all monitoring points are calculated. Fractal dimension criterion is presented. Hurst indexes of displacement-time series of all measurement points are greater than 0.5 and less than 1, and Hurst indexes of displacement-time series in cross-section neighboring from M410 approximate 1, which shows that the deformation of surrounding rock will continue to grow up and its increasing rate keeps accelerating. From one cross section to another, fractal dimension of displacement time series collected by multipoint extensometer in two shallower measurement points(orifice、5.0 m) increase and then decrease, and fractal dimension in fractured cross-section where M410 is installed is least, which indicates that the occurrence of dimension minimum is a alarm for the forthcoming cracking of surrounding rock. However, dimension in two deeper measurement-points(11.0 m，17.5 m) do not have the characteristics similar to the one in two shallower measurement points. It is showed that crack depth in the cross-section where M410 is installed should be greater 5.0 m and less than 11.0 m.