Abstract: Niuchang ancient underground caverns near Shangfan Village,Longyou County are composed of two caverns.Through field investigation,a vertical tensile crack was found in the No.1 rock pillar of No.1 cavern,which resulted from the overloading of upper rock layers.This paper presents the back analysis of the rock mass long-term tensile strength according to tensile cracks in the No.1 pillar,which not only helps to protect ancient engineering sites but also benefits the theoretical research on the long-term strength of rock mass.