<p>This study systematically investigates the pressure dependence of P-wave velocity anisotropy in stressed shale through water injection experiments. A modified single-plug method (MSPM) based on iterative reweighted least squares is proposed to accurately determine orthorhombically anisotropic velocities and parameters under anisotropic stress conditions. Triaxial compression tests with water injection on Longmaxi shale (Sichuan Basin) reveal that both isotropic and differential stress fields significantly influence velocity anisotropy, exhibiting nonlinear relationships with stress and pore pressure. Due to anisotropic Biot coefficients, water injection induces asymmetric pore pressure gradients, leading to distinct mechanical responses such as stress hysteresis and secondary failure during unloading. Results highlight that anisotropy parameters like δy are highly sensitive to stress changes, while velocity in the direction parallel to bedding(Vz) shows pronounced hysteresis. Adsorption of injected water at clay particle interfaces is identified as an additional factor enhancing anisotropy beyond stress effects. This study deepens the understanding of stress-dependent shale anisotropy, emphasizing the critical role of pore pressure dynamics in reservoir characterization and hydraulic fracturing design for unconventional oil and gas development.</p>