Carbonized polymer dots (CPDs) have attracted great interest as a nanomaterial in biomedical applications, especially in imaging and photodynamic therapy (PDT). However, CPDs with an aggregation-induced effect for type I PDT has been rarely reported. In this work, N,N′-dimethylurea and citric acid were used as precursors to prepare CPDs (YCPDs) with aggregation-induced effect in one pot by the solvothermal method. The YCPDs have a yellow emission of 550 nm and a high quantum yield of 68%, which lays a solid foundation for cell imaging. In addition, YCPDs can produce •O2– under white light irradiation and obtain •OH through electron transfer or hydrogen atom abstraction. In vitro results show that YCPDs possess great cytocompatibility but exert severe cytotoxicity to HeLa cells with only 2% of cell viability when exposed to ultralow-power-intensity (2.5 mW/cm2) light irradiation. Therefore, YCPDs, as a promising type I PDT nanomaterial, have a great potential for cancer therapy.