Polybenzoxazine (PBa) aerogels have gained much attention as a fire‐retardant material because of their flexible molecular design and excellent mechanical properties. Incorporation of inorganic fibers into PBa aerogels using a simple and effective method seems attractive yet challenging. Herein, we report a unique method for the one‐step preparation of basalt fiber (BF) reinforced PBa composite aerogels by freeze‐drying using carboxymethyl cellulose (CMC) as a dispersion carrier. Aqueous benzoxazine (Ba) monomer was used as the matrix and phosphoric acid was used as the polymerization catalyst, while the uniform dispersion of BF was achieved by blending with CMC. The results indicate that the PBa/CMC/BF composite aerogel reveals a layered microstructure with BF dispersed between the layers. And the composite aerogel achieves a low volume shrinkage (1.11%), and strong mechanical strength (compressive stress of 1.73 MPa at 60% compressive strain). Additionally, TGA analysis reveals that the composite aerogel achieves a higher initial decomposition temperature (> 200°C) and residual mass (> 48 wt%) in a nitrogen atmosphere. Excellent flame retardancy of PBa/CMC/BF composite aerogel is achieved due to the introduction of nonflammable BF: as BF content rises, its peak heat release rate decreases by 92.5%, average heat release by 86.6%, and total smoke release by 76.7%. In combustion tests, the composite aerogel self‐extinguishes within 0.5 s. After hydrophobic modification, the composite aerogel achieves a water contact angle greater than 135°, while PBa/CMC/BF3 exhibits the typical “silver mirror phenomenon” in water. This study presents a straightforward and rapid method for constructing a lightweight, flame retardant, hydrophobic, and high‐strength fiber‐reinforced PBa composite aerogel, with promising applications in the aerospace sector.
