<p>Deep underground excavations in tectonically active regions face severe threats from the violent fault-slip bursts. Whether a fault-slip burst occurs highly depends on the shear stiffness of the slipping joints. However, the joint shear stiffness evolution over cumulative shear displacement remains poorly explored, particularly when progressive asperity degradation and fault gouge interplay in the post-peak stage. Here, we experimentally examined the post-peak joint shear stiffness change via multiple loading-unloading shear tests on sawtooth sandstone joints under low to high normal stresses by integrating the high-resolution digital image correlation (DIC) and strain gauge arrays. Our laboratory results showed that joint shear stiffness neither keeps constant nor decreases monotonically but exhibits distinct evolving patterns dictated by the asperity failure mode. In the shear-off after climbing failure, the joint shear stiffness decreases and partially recovers due to adjustments in asperity slope; whereas in the pure shear-off failure, joint shear stiffness declines and subsequently stabilizes as the asperities are completely sheared off and fault gouge redistribute. A higher normal stress elevates the joint shear stiffness via accelerated shear stress transfer but promotes the development of cracks leading to rock rupture and accompanied joint shear stiffness reduction. Additionally, lubricative fault gouge lower the joint shear stiffness whereas their compaction significantly affects stiffness recovery. Our findings facilitate the prediction of fault-slip bursts in deep underground rock engineering based on stiffness-driven mechanisms.</p>