Biochar is widely used in water treatment because of its porous structure, however, the effects of biochars on denitrification remain unclear. Here, we combined molecular biological and electrochemical techniques to investigate effects of biochars (formed at 300 degrees C, 500 degrees C and 800 degrees C) on denitrification. Results showed that biochar at 300 degrees C increased total nitrogen removal by 415% and decreased N2O accumulation by 78%. Mechanistic research demonstrated that it achieved the highest electron transfer efficiency and denitrifying enzyme activities. Further study evidenced that biochar at 300 degrees C increased the abundance of denitrifiers such as Pseudomonas. Correlation analysis indicated that nitrate reductase and nitrite reductase activities were the key factors influenced by biochar during denitrification. Overall, this study suggested that biochar at 300 degrees C could act as the bio-engineer of electron shuttle and the stimulator of denitrification, achieving high rate nitrogen removal and significant reduction of N2O accumulation from high-strength wastewater.