Abstract: As its occurrence conditions changes, the quality of rock mass has corresponding dynamic change. So the dynamic evaluation of rock mass quality for dam foundation is of great significance for the safety of dam operation. This paper is based on a large number of tests and monitoring data of the foundation rock mass at Xiaowan Dam. It summarizes the dynamic characteristics of the quality of rock mass and then puts forward the evaluation index of rock mass qualitys degradation and recovery. It also uses the BQ system and RMR system to evaluate dynamically the characteristics of the change of the rock mass quality in the four-dimensional space. Furthermore the relationship between Vp and E0(Vp-E0) is established and the Hoek-Brown strength criterion is adopted to determine the strength index of the feature points of the rock mass. At the same time, the relaxation degree coefficient of rock mass is used as indicators to examine a kind of simple and easy forecasting method of rock mass quality.