<p>Longyou Grotto argillaceous siltstone is highly sensitive to water infiltration, and its strength degradation poses a serious threat to the stability of grotto chambers and underground engineering. In this study, in situ uniaxial compression tests combined with high-energy CT real-time scanning and PFC2D numerical simulations were conducted to investigate crack evolution under different moisture distributions. Stress–strain curves and fracture morphologies obtained from CT experiments were used to calibrate the numerical model. Experimental results show that the strength–moisture relationship is nonlinear. When the moisture content is below 50%, strength decreases with increasing water content, whereas when the moisture content exceeds 50% and approaches saturation, the strength reduction becomes less significant. The numerical simulations reproduced the entire loading process, capturing strength characteristics and fracture patterns consistent with CT observations, and revealed that crack evolution and acoustic emission (AE) features also vary systematically with moisture content. Higher moisture levels promote shear rather than tensile cracking, with vertical fractures predominating, and AE activity shifting toward more numerous, smaller, and more dispersed events. Both experimental and numerical results further demonstrate a distinct transition in fracture patterns: from single inclined shear failure under low moisture, to X-shaped conjugate shear failure under moderate moisture, and finally to splitting-induced surface spalling and internal shear damage near saturation. These findings clarify how moisture content governs the strength and fracture mechanisms of argillaceous siltstone, providing new insights for the preservation and long-term stability assessment of water-affected underground grottoes.</p>