Abstract: Casing shear deformation, induced by frictional sliding along weak planes in shale formations, represents a critical constraint on the safe and efficient extraction of deep shale gas. To elucidate the frictional sliding characteristics and the associated mechanical response mechanisms, a temperature-pressure coupled triaxial friction testing system was employed. Shale specimens extracted from the Longmaxi-Wufeng Formations in the Sichuan Basin were tested under simulated in-situ reservoir conditions, with temperatures ranging from 100 to 160 ℃, confining pressures from 105 to 115 MPa, and pore pressures from 80 to 100 MPa. The effects of weak-plane inclination angle (30°and 60°), pore fluid composition (neutral water and acidic solution), and loading rate on the friction coefficient and critical slip pressure were systematically analyzed. The results indicate that temperature exerts a negligible influence on the friction coefficient; however, elevated temperature (160 ℃) substantially reduces the sliding stability of the weak planes. An increase in confining pressure significantly elevates the critical axial slip stress (from 125.94 MPa to 141.41 MPa for the 60° weak plane) but concurrently lowers the friction coefficient (from 0.8987 to 0.5736) due to the enhanced effective normal stress, while the dispersion of the rate-dependent parameter (a−b) diminishes with increasing confining pressure. Elevated pore pressure weakens the shear strength of the weak planes, with a maximum reduction of 44.5% in the critical axial stress. Acidic solutions raise the threshold of the friction coefficient through mineral dissolution, yet they concurrently intensify sliding instability. The inclination angle governs the slip initiation behavior: the critical pore pressure for the 60°weak plane (94−96 MPa) exhibits negligible sensitivity to temperature variations, and high-angle weak planes are more prone to inducing casing shear deformation. In contrast, the 30°weak plane requires extreme mechanical conditions specifically, a confining pressure of 115 MPa and a pore pressure of 106 MPa to initiate slip, during which a creep-rupture competition mechanism is observed. The rate-dependent behavior of the friction coefficient was characterized; weak planes with high inclination angles predominantly exhibit velocity-strengthening behavior, whereas those with low inclination angles are susceptible to instability induced by decreasing loading rates. Based on these experimental findings, a series of casing protection and optimization strategies are proposed, including modulation of the friction coefficient (μ > 0.5), geometric optimization of the wellbore trajectory (intersection angle with weak planes > 50°), and staged fracturing pressure control (maintained below 96 MPa). These measures will provide a theoretical foundation for the safe and efficient development of deep shale gas resources.