Abstract:On the basis of the analysis of process of excavation unloading and the calculation of its releasing duration,it is shown that the excavation unloading during rock blasting is a transient process under middle to high in-situ stress conditions. It is proposed that the transient characteristics of the excavation unloading and correspondingly induced dynamic responses of surrounding rock should be considered under middle to high in-situ stress conditions. Problems such as the determination of divisional excavation unloading corresponding to millisecond delay initiation,the path of transient releasing of excavation unloading,releasing of excavation unloading-induced vibration and the mechanism of dynamic loosening of jointed rock mass induced by the transient unloading of in-situ stress,etc. are also discussed. Finally,two historical cases of excavation unloading-induced vibration and dynamic failure of jointed rock mass in Pubugou and Ertan Hydropower stations are presented as verifications.