Abstract: Under the background of replacing plastic with bamboo and double carbon, the use of micron fiber modified polymer prepared from waste bamboo to replace traditional plastic will effectively help green carbon sequestration and sustainable development. The preparation of micron fiber modified polymer is often carried out by dry mixing extrusion process, that is, the dry micron fibers are directly extruded with polymer and coupling agent. However, the micron fibers in the obtained composites are easy to agglomerate and have poor dispersion. In this study, the wet mixing extrusion process was used, that is, the bamboo fiber was treated with 2wt% NaOH solution and washed to neutral, and then added with water in a high-speed mixer to defiber into micron bamboo fiber (BMF), and then filtered to a non-dripping state to obtain wet pulp. Finally, the wet pulp was blended with PP and coupling agent (maleic anhydride grafted polyethylene) and extruded. When the amount of BMF added is 10wt%-60wt%, the composite material has good fiber dispersion and interface bonding effect. The microstructure, mechanical properties and thermal expansion properties of the composites prepared by wet mixing extrusion were studied, and the composites prepared by dry mixing extrusion were used as the control group. The results show that when the addition amount of BMF reaches 50wt%, the tensile strength and flexural strength of the samples prepared by wet method are as high as 37.8 MPa and 53.04 MPa, which are 19.92% and 8.31% higher than those of the samples prepared by dry method, respectively. In the temperature range of −30~60℃, when the addition amount of BMF reaches 60wt%, the thermal expansion coefficient of the composite prepared by wet method is as low as 31.4×10⁻⁶/K, which is 11.9% lower than that of the sample prepared by dry method. In addition, compared with the dry method, the BMF/PP composites prepared by the wet method showed better dynamic mechanical properties at the same BMF addition.