Biochar, as an unique soil amendment, can effectively improve soil structure, improve the ability of absorbing nutrients, reduce greenhouse gas emissions and increase biological nitrogen fixation. Therefore, it has great prospects in agriculture production and mitigating climate change. Biochar input will directly affect the agricultural soil nitrogen cycle and transformation, so this review focuses on the effects of biochar on soil nitrogen transformation processes such as nitrogen mineralization, nitrification, denitrification, the relevant nitrogen loss, and biological nitrogen fixation processes and relevant mechanisms. Based on the review on published literature, we proposed that the future research should strengthen the mechanism nitrogen transformation and the long-term positive or negative effects of biochar on environment, to carry out research on related microbial community diversity, abundance and activity in terms of soil, as well as biochar studies should be based on a unified standard to clearly distinguish the effect of different biochar and its mechanism.