Abstract: Biocarriers play a critical role in the biofilm process, so they are essential in wastewater treatment. Commercial biocarriers have inherent defects such as poor bioaffinity and inefficient biofilm formation. It is thus important to develop new carriers to improve wastewater treatment efficiency to benefit the application and development of biofilm reactors. The polyurethane with bioaffinity (BPU) was prepared by physical coating method, and the apparent characteristics and chemical composition of the carrier before and after modification were analyzed and compared. Then, the new carrier was applied in a moving bed biofilm reactor as a filler to treat simulated domestic wastewater. Its bioaffinity and wastewater treatment efficiency were evaluated. In addition, the synergistic mechanism was analyzed by comparing the biofilm growth and the changes in microbial community structure. The results showed that: (1) The modification increased the surface roughness and specific surface area of the carrier. The zeta potential of the carrier surface changed from (−31.70±1.93) mV to (2.14±0.14) mV. The contact angle decreased from 44.50°±0.14° to 24.88°±1.46°. (2) After being applied to the biofilm reactor, the biomass attached to the surface of the BPU and the secretion of extracellular polymer substances (EPS) increased, the composition of biofilms was improved, and the removal rate of TN and NH₄⁺-N increased by 6.27%±0.30% and 13.74%±0.68%, respectively. (3) Microbial analysis showed that the BPU enriched the diversity of the microbial communities, increased the abundance of denitrification functional bacteria, and promoted the removal of contaminants. Therefore, the BPU has a good bioaffinity for improving the performance of biofilm reactor.