Abstract: The red-bed rock mass, prevalent in Southwestern China, often leads to geological hazards like landslides. Understanding its mechanical behavior is crucial for prevention. Through a combination of indoor tests and numerical modeling, the red-bed rocks from the Majiagou landslide area have been extensively studied. Specifically, indoor triaxial compression tests have been conducted to obtain the stress-strain curves of the rock samples. Additionally, numerical rock samples, generated using particle flow code (PFC),have undergone calibration according to experimental results. Based on numerical simulations of lateral confined compression tests, differences in stress-strain curves, poisson's ratios, failure modes, and microscopic particle contact forces of the rock samples have been analyzed under both rigid and flexible boundary conditions. The results indicate that the rigid boundaries strongly constrain rock samples, causing elastic modulus and peak strength to rise rapidly with increased confining pressure. At the same pressure, the Poisson's ratio remains stable for flexible boundary samples pre-peak, but tends to increase for rigid boundary samples. The Poisson's ratio at peak is notably higher under rigid conditions. Rigid boundary samples often fail by axial splitting or diagonal shear, with tension and shear damage prevailing. Flexible boundary samples mainly fail by shear damage, showing lateral dilatation. At lower confining pressures, flexible boundary samples show marginally higher shear and tensile stresses. These stresses significantly surpass those of rigid boundary samples as confining pressure rises, but converge at higher pressures.