Taking the tail water of a large wastewater treatment plant in Xinxiang city, Henan Province of China, as the research object, the deep nitrogen and phosphorus removal by constructed wetlands (CWs) for this tail water was studied. Polyhydroxybutyrate valerate (PHBV) and pyrite were selected as the main functional fillers of wetland system to construct a mixotrophic denitrification system by combining heterotrophic denitrification and autotrophic denitrification. Water quality index monitoring and high-throughput sequencing technology were used to explore the removal performance and microbial community structure. The PHBV+pyrite experimental group and PHBV+ceramite control group were set up for continuous operation for 77 days. Results show that when hydraulic retention time (HRT) was 2 hours, NO₃⁻-N, TN, TP and COD concentration are 8.39±1.72 mg/L, 10.41±1.58 mg/L, 0.37 mg/L and 15.7 mg/L, respectively, the average effluent TN concentration reaches at 2.22 mg/L, TP concentration reaches 0.28 mg/L and COD concentration is 32.7 mg/L. The nitrogen removal efficiency of CWs in PHBV+ pyrite group (78.55%) is obviously better than that in PHBV+ceramite group (51.25%), basically meeting the surface water environmental quality standard (GB 3838–2002) Class V. Analysis of the microbial community structure shows that the PHBV + pyrite group advantage bacterium in artificial wetland are autotrophic bacteria Desulfobacter and heterotrophic bacteria unclassified_k__norank_d__Bacteria, the abundance are 23.98% and 12.30% respectively. Mixotrophic denitrification is achieved through coordination of autotrophic and heterotrophic bacteria, in which autotrophic denitrification led by Desulfobacter related to sulfur metabolism may play a major role in systematic nitrogen removal. These results can provide reference for the engineering practice of biological nitrogen and phosphorus removal in tail water of PHBV+pyrite-based CWs.