Abstract: As a kind of multifunctional eco-friendly material, biochar has been widely used in sewage treatment by constructed wetlands in recent years, which can provide carbon source for heterotrophic denitrification and enhance nitrogen removal capacity of constructed wetland. To explore the effect of external carbon addition on deep purification of the tail water of a sewage treatment plant by biochar-based horizontal subsurface flow constructed wetland, two parallel indoor experimental units were set with quartz sand/almond shell biochar (volume ratio 7∶3) and quartz sand (the control) as the constructed wetland matrix, respectively. Moreover, in the later phase of operation, different C/N ratios were designed by external carbon addition and the operation modes of continuous flow and intermittent flow were adopted, respectively. Results showed that before external carbon addition, chemical oxygen demand (COD) removal rate of constructed wetland was negative, and total nitrogen (TN) and nitrate nitrogen (NO₃ ⁻-N) removal rates continued to decrease during 41 days. However, COD removal rates of quartz sand and quartz sand/almond shell biochar units increased to 37.88%-90.44% and 73.60%-97.90%, respectively, and TN and NO₃ ⁻-N removal rates also significantly increased after external carbon addition. The maximum removal rates of TN and NO₃ ⁻-N in quartz sand/biochar unit were 65.61% and 74.20%, respectively, as the influent C/N ratio was increased to 8 by external carbon addition and the intermittent flow mode was running. Biochar addition increased the microbial biomass of the constructed wetland, and created the appropriate redox environment facilitating denitrification. Therefore, the removal rates of COD, TN, and NO₃ ⁻-N of the wetland unit set with quartz sand/almond shell biochar increased by 5.66%-130.35%, 9.34%-54.03%, and 8.71%-63.04%, respectively, compared with the control. The external carbon addition and biochar application could be an effective measure to strengthen the denitrogenation efficiency of constructed wetland for tail water purification of sewage treatment plants.