Hexagonal BN particles (BNp) reinforced fused SiO2 matrix composite (BNp/SiO2) was regarded as a very important candidate material for potential applications on wave transparent thermal protection parts. In this study, pressurelessly sintered 15 vol% BNp/SiO2 composites were prepared by cold isostatic pressing (CIP) and gel-casting routes, respectively, using two kinds of fused silica powders with different particle sizes (5.82 and 3.24 μm in d50, respectively) as starting SiO2. It was demonstrated that the microstructure and mechanical properties of the composites were well dependent on the preforming techniques of green compacts. Gel-casting preforming route has an overwhelming superiority over CIP route on the thermal stability of fused SiO2 matrix against α-cristobalite crystallization, sinterability, and thus the mechanical properties. Using the finer fused silica (3.24 μm in d50) as the starting SiO2, the 15 vol% BNp/SiO2 composite sintered at 1375°C get a density of 2.05 g/cm3, the silica matrix primarily keep amorphous phase with uniformly dispersed nanosized α-cristobalite crystallites (<5 nm in diameter). The bending strength, fracture toughness and Young's modulus all reach the highest values, 101.5 ± 4.3 MPa, 1.57 ± 0.04 MPa·m1/2 and 61.3 ± 2.4 GPa, respectively. Microstructure characteristics, chemical bond information and mechanical properties of the as-prepared composites are correlated with their preforming routes, starting fused SiO2 particle size, sintering temperature, etc. based on X-ray diffractometry, scanning electron microscope, transmission electron microscope, high resolution transmission electron microscope, and FT-IR analysis.