In order to improve the mechanical properties and thermal stability of nylon 66 (PA66) and make it widely used in the field of automotive lightweight, 5%~20% carbon fiber (CF) was doped in PA66 matrix. The results show that the PA66 composite doped with 15% CF has better comprehensive properties than the unmodified PA66 material. The universal testing machine test shows that the tensile strength of PA66/15% CF is 180 MPa, increased by 177%, and the elastic modulus is 19 867 MPa, increased by 7.1 times. The bending strength is 262 MPa, which is increased by 212%, and the bending modulus is 15 867 MPa, increased by 6.4 times. The impact strength of PA66/15%CF is 12.7 kJ/m², which increases by 49.4%. The mass residual rate of PA66/15%CF composite is 22.8%, and the thermal stability of the material is significantly improved. The study indicates that CF-modified PA66 composites have the potential for use in the field of automotive light weighting, with a 15% doping amount being preferable.