Instantaneous unloading is instantaneous rock mass stress release and adjustment caused by rock mass excavation. According to the instantaneous unloading process of tunnel rock mass excavation in high stress zone, the instantaneous unloading displacement formula was deduced through the energy conservation; the initial fracture and the complete rupture time of tunnel wall rock mass unloading were deduced based on brittle failure theory of Kachanov criterion, and it was obtained that the complete rupture time is equal to the completion time of rock mass excavation unloading. By the analysis on the instantaneous unloading of a tunnel in Jinping Ⅱ hydropower station, it was concluded that rock blasting would be caused under instantaneous excavation unloading conditions and the initial fracture time was shorter than the complete rupture time, but the fracture has been very clear at that time.