Developing a low-cost photocatalyst with efficient performance is significant for practical application of solar-to-fuel conversion. Here, we first adopt a facile method to synthesize Bi2O2CO3-modified g-C3N4 heterojunction via in situ thermal growth. Bi2O2CO3 nanoparticles on g-C3N4 nanosheets play a vital role in improving the photocatalytic activity of splitting water for hydrogen production. The activity of Bi2O2CO3/g-C3N4 heterojunction during 5 h reaches 965 mu mol.g(-1).h(-1), which is much higher than that of pure g-C3N4 (337 mu mol.g(-1).h(-1)) or other modified g-C3N4 materials. The significantly enhanced photocatalytic activity is attributed to direct Z-scheme system construction, resulting in a superior charge carrier separation ability. Theoretical calculations further reveal the redistribution of charge carrier at interface between Bi2O2CO3 and g-C3N4. This work provides new direction to synthesize g-C3N4-based heterojunction with high photocatalytic performance for alleviating energy and environmental issues.