Many constructed wetland systems are facing the problem of low dissolved oxygen (DO) and reduced nitrogen removal efficiency. In this study, an experimental constructed wetland system is designed and used to investigate the effect of biochar (rice husk biochar (RHB), coconut shell biochar (CSB), and wood biochar (WB) and earthworm on DO concentration, nitrogen transformation, and ammonia nitrogen removal. Specifically, effects of different biochar and earthworm on NH-N in wastewater, N content of Phragmites australis, NH-N and NO-N content in substrates, microbial nitrification and denitrification potentials, and the DO concentration were investigated. Results show that the addition of biochar and earthworm increased the removal efficiency of NH-N from wastewater. The addition of RHB and WB significantly increased the concentration of DO by 21.4% and 25.7% (P < 0.05) respectively in the constructed wetland. The addition of earthworm significantly increased the DO concentration in the constructed wetland system by an average of 30.35% (P < 0.05).The N content of P. australis increased when biochar and earthworm were introduced into the constructed wetland system, with higher relative N content observed in the above-ground biomass. NO-N content increased, but NH-N decreased in the substrate. Addition of both biochar and earthworm increased nitrification and denitrification potentials. However, no significant increase in denitrification potential was observed when only biochar was added. The removal efficiency of NH-N from wastewater is significantly positively correlated with the DO, nitrification, and denitrification potential, and nitrogen content of above-ground part of P. australis (P < 0.05). Results suggest that the DO concentration in constructed wetland systems could be improved by the addition of biochar and earthworm. These findings imply that both biochar and earthworm could be added into constructed wetlands to solve the low DO concentration and improve the removal efficiency of nitrogen.