Abstract: Continuous basalt fiber-reinforced polyamide 6 (BF/PA6) composite laminates were prepared via injection/compression liquid composite molding (I/CM) combined with in situ anionic ring-opening polymerization. The effects of molding pressure and fabric areal density on the morphology and structure of the reinforcement, the distribution of void defects, and the mechanical properties of the composites were investigated. The results showed that the in situ polymerized PA6 matrix exhibited good thermal stability, with the temperature corresponding to the maximum weight-loss rate reaching 415.02℃. For composites reinforced with fabrics of 200 g/m² areal density, the fiber volume fraction gradually increased with increasing molding pressure, whereas the void content first decreased and then increased; the mechanical properties were jointly affected by the void content, fiber volume fraction, and interfacial bonding state. The composite prepared at a molding pressure of 0.61 MPa exhibited better mechanical properties, with the flexural strength, flexural modulus, and interlaminar shear strength reaching 424.61 MPa, 24.43 GPa, and 33.52 MPa, respectively. At a molding pressure of 0.61 MPa, when the fabric areal density increased from 200 g/m² to 300 g/m², the void content determined from cross-sectional images increased significantly, while the flexural properties, interlaminar shear properties, and dynamic thermomechanical properties all decreased. Analyses of the composite cross-sectional structure and the fracture morphologies of flexural specimens indicated that, within the range of plain-woven fabric parameters investigated in this study, higher fabric areal density and yarn linear density were unfavorable for monomer penetration into the fiber bundles and gas removal during processing, whereas appropriate fabric areal density and molding pressure were beneficial for improving resin impregnation, reducing internal defects, and enhancing the mechanical properties of the composites. This study provides a reference for optimizing low-viscosity monomer impregnation and in situ polymerization processing of continuous fiber-reinforced thermoplastic composites.