Biochar, the solid product of biomass pyrolysis, has been widely used to mitigate global climate change by storing carbon (C) and regulating the transformation of nitrogen (N) in soil. While the effects of biochar on soil C and N cycles, such as mineralization and plant uptake, have been extensively reported, most studies of these types have employed measurement of the changes in total C and N in soil following biochar addition. However, this does not allow for the determination of the sources of C and N in soil that are utilized or transformed. Isotope technology has great potential in evaluating the effects of biochar on the biogeochemical behavior of soil C and N because it can distinguish added biochar C and N from native C and N and other sources of organic matter and determine the transformation or loss rate of each source. However, at present, most studies on this aspect are scattered and the conclusions are inconsistent. Therefore, a systematic review of studies using the C and N isotopes in biochar research is needed. In this paper, the advantages and disadvantages of the application of C and N isotopes to biochar research are summarized, and the findings of these studies as related to the effects of biochar on soil C and N cycles were reviewed, including biochar and native C persistence, CO₂ and N₂O emissions, and plant N uptake. Finally, some recommendations for future directions on C and N isotopes in the field of biochar research are made. This review could lay a theoretical foundation and encourage the application of C and N isotope technology to the field of biochar research.