Seepage leaching increases the risk of spalling in hydraulic tunnel concrete linings. This study first analyzed the spalling model and selected the joint surface (single fracture) between the spalled and unspalled sections of the lining as the research subject. The improved prefabrication method of fractures based on three-dimensional (3D) scanning, 3D modelling, and 3D printing technology was then used to replicate the fracture surfaces of the actual hydraulic tunnel lining, followed by seepage leaching tests. Third, the parameters of the lining spalling model are obtained through improved L-shaped shear tests. The tests revealed an average decrease of 19.2 % in the shear capacity of fractures after seepage leaching. Finally, the safety factor against spalling (SFAS) before and after leaching was quantified based on the spalling model and experimental parameters. The results showed a decrease in SFAS due to leaching. Additionally, this study proposed that the degradation of calcium asperities on fracture surfaces was the cause of the decrease in shear capacity after leaching by analyzing the microscopic leaching of minerals on fracture surfaces in combination with petrographic observations. The aim of this study is to provide theoretical guidance for the maintenance of hydraulic tunnels during operation.