Abstract: In order to further enhance the ability of denitrification partial nitrification and anaerobic ammonia oxidation (DN-PN-ANAMMOX) process for the treatment of mature landfill leachate containing high NH₄⁺-N, the local landfill leachate was used as test water to remove organic matter and nitrogen. Meanwhile, under different recycle ratios, the change of microbial communities was thoroughly investigated. The results showed that the removal efficiency of COD_Cr (chemical oxygen demand) gradually increased from 47% to 53% as the recycle ratio increased from 5 to 9. The COD_Cr removed by UASB was 41%, providing a low-carbon environment for the subsequent PN-ANAMMOX process. However, with the increase in the reflux ratio to 9, the dilution effect made the concentration of FA (free ammonia) lower than 2 mg/L, which cannot suppress NOB (nitrite oxidizing bacteria) activity in the PN zone. The stability of partial nitrification decreased and the nitrite accumulation efficiency decreased to less than 50%, which was harmful to the subsequent anammox zone. Therefore, the nitrogen removal rate of the anammox zone decreased to 4.29 kg/(m³·d). The research proved that the optimal recycle ratio was 8, and DN-PN-ANAMMOX process with recycle treated the mature leachate with 1900 mg/L NH₄⁺-N successfully. The nitrogen removal rate of anammox region reached 7.41 kg/(m³·d), and the effluent total nitrogen was the lowest (8.83 mg/L). When the recycle ratio was 8, the optimal enrichment of Pseudomonas, Nitrosomonas and Candidatus_Kuenenia was achieved in the UASB, PN and ANAMMOX zones, respectively.