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[Display omitted] • The preparation methods influenced properties of biochar used in CWs. • Biochar addition improved transformation of pollutants in CWs via various pathways. • Biochar addition significantly enhanced common and emerging pollutants removal in CWs. • A novel Fe-C micro-electrolysis substrates boosted treatment performance of CWs. Traditional constructed wetlands (CWs) still face tremendous challenges, including low nitrogen removal, unstable operation, undesired refractory substances degradation and poor reoxygenation capacity, in their practical applications. Recently, biochar has been regarded as a promising alternative substrate in CWs to enhance pollutants removal via inner environment adjustment, plants growth facilitating and biotoxins adsorption. Based on VOSviewer analysis, this study comprehensively summarizes the mechanisms, application, and performances of biochar addition in CWs. First, physicochemical properties of biochar, including pore sizes/pore volumes, surface aeras, functional groups and chemical compositions, with different preparation methods were comparatively analyzed. Then, the mechanisms and applications of (modified) biochar in CWs and its improving effects on typical pollutants (such as nitrogen, phosphorus, complex organics and heavy metals) removal were summarized. Moreover, the mechanisms and strengthened effect of novel Fe-C micro-electrolysis substrates on pollutants reduction in CWs was also elucidated. Finally, the prospectives and challenges were proposed to offer a deep insight in biochar for its engineering application in CWs. [ABSTRACT FROM AUTHOR]