To explore the feasibility of basalt fiber (BFs) as filler to enhance engineering plastics, BF-filled polyamide 66 (PA66/BFs) composites were prepared by a twin-screw co-extrusion machine. The effects of BFs mass fraction (26% , 34% , 42% and 50% ) on the internal fiber morphology, crystallization behavior of the resin matrix, mechanical properties and thermal properties of the composites were investigated in detail, and the performance enhancement mechanism of basalt fiber modified PA66 was clarified. The results show that with the increase of short-cut BFs content, the fiber length in the composites gradually decreases, and the ordered arrangement of the resin molecular chain is hindered, resulting in a gradual decrease in crystallinity. The mechanical properties of composites change in a positive correlation with the increase of short-cut fiber content. With the increase of reinforcing fiber content, the continuity of the rigid reinforcing skeleton formed inside the composites gradually increases; not only can it withstand higher external loads, but also the ability to inhibit the generation and expansion of internal cracks in the composites is significantly improved. When the BFs mass fraction reaches 50% , the tensile strength of the composite can reach 192 MPa and the flexural modulus can reach 356 MPa. In addition, the existence of inorganic rigid skeletons can significantly improve the heat resistance of PA66, and the heat distortion temperature can reach 241.8 ℃.