Organic conjugated polymers have received extensive attention due to their unique electronic properties. However, there have been relatively few reports on the dark photocatalytic reactions utilizing organic conjugated polymers. Herein, we report the successful synthesis of an organic conjugated polymer based on poly (heptazine imide) nanocrystals (CNNCs) for H₂O₂ evolution and biomedical applications using a simple salt molten method and sonication–centrifugation process. The results show that these colloid CNNCs have the characteristics of photogenerated electrons accumulation and realize dark photocatalysis with high reducibility under visible light irradiation. Notably, these accumulating photogenerated electrons can reduce O₂ in darkness to produce H₂O₂. In addition, cytotoxicity tests were conducted and it was found that H₂O₂ produced under dark conditions could oxidize L-arginine (L-Arg) to NO, which effectively killed tumors in the dark. This work provides an important strategy to construct organic conjugated semiconductor nanocrystals and applying them to future energy and biomedical fields.