Photothermal therapy (PTT) is a minimally invasive treatment that kills cancer cells by converting photon energy into heat.The past few decades have witnessed the booming development of photothermal materials,mainly focusing on precious metal nanomaterials and carbon nanomaterials,such as nanogold and silver and nanocarbon materials for near-infrared (NIR) light-triggered PTF.As precious metals are expensive and potentially harmful to humans,exploration and development of a new type of photothermal materials has become a research hotspot in this field.Herein,we report narrow bandgap conjugated polymer nanoparticles (PDPP NPs) based on pyrrolo[3,4-c]pyrrole-1,4-dione (DPP) with intense NIR absorption at 900 nm,as well as a photothermal energy conversion efficiency of 75％.This polymer nanoparticle is essentially non-toxic,as the cell viability of mouse remained more than 90％,even when the concentration of PDPP NPs was at 0.5 mg·mL-1.