<p>This study presents a systematic investigation on the material design, preparation, mechanisms, and shear strengthening application of basalt fiber reinforced cementitious composites (BFRCC). Unlike conventional composites relying on synthetic fibers, the proposed BFRCC utilizes basalt fiber (BF) as the sole reinforcement, offering advantages of low cost and low carbon footprint, with potential for improved fire resistance. The main conclusion can be obtained: (1) Through systematic optimization of fiber parameters and matrix composition, the tensile properties of BFRCC exhibit superior designability: the tensile strength can be tailored within a wide range of 3–8&#xa0;MPa, and the ultimate tensile strain within 0.1–0.6%. (2) Basalt fiber exhibits superior compatibility with fly ash-dominated matrices compared to cement-dominated matrices. Reduction of water-to-binder ratio (W/B) from 0.20 to 0.16 and increase of sand-to-binder ratio (S/B) from 0 to 0.2 enhance tensile strength but compromise ultimate tensile strain. In addition, shorter fibers and higher fiber dosage further improve the tensile strength of BFRCC. (3) The underlying mechanism is matrix-dependent fiber-matrix interaction governed by interfacial densification. In cement-dominated matrices, dense interfaces cause isolated fiber failure. In fly ash-dominated matrices, moderate interfacial porosity enables fiber slip, crack path deflection, and multi-fiber synergy. Optimizing BFRCC therefore requires appropriate interfacial weakening to transform failure from sequential to collaborative fiber engagement. (4) BFRCC offers a cost-effective and eco-friendly solution for masonry strengthening. Diagonal shear tests demonstrated a 3–fivefold increase in shear strength and enhanced structural integrity, validating the integrated structures and materials design approach where material-level parameters are tailored to achieve targeted structural performance.</p>