Fractured rock masses are widely present in geological and engineering settings, and their microcrack evolution and propagation mechanisms directly affect structural stability. To investigate these mechanisms in defective rocks, this study establishes a numerical model of pre-fabricated parallel double-fissure red sandstone using particle flow code (PFC2D) to simulate microcrack initiation, propagation, and coalescence under different confining pressures. The results show that increasing confining pressure causes the failure mode to shift from tensile splitting to shear failure, primarily governed by the coalescence of tensile microcracks. Reverse wing cracks form at fissure tips and evolve into reverse shear planes under high confining pressure. The number of tensile microcracks at peak stress increases in a stepwise manner, with critical thresholds identified at 0 MPa, 10 MPa, and 20 MPa. In contrast, shear microcracks are more sensitive to confining pressure and initiate more readily at higher pressures, though their total number remains significantly lower. Numerical analysis reveals that stress concentration at fissure tips, compression-induced tensile effects, and the deflection of contact force directions are the main drivers of microcrack development. This study provides a theoretical foundation for understanding and predicting fracture behavior in defective rock masses and offers valuable insights for engineering applications such as tunneling, slope stability, and underground excavation, where confining pressure critically influences rock failure mechanisms.