Chemiluminescence (CL) is a promising bioimaging method due to no interferences of light source and autofluorescence. However, compared to fluorescent emission, most CL reactions show short emission time and wavelength and weak emission intensity, which limit their applications in in vivo imaging. Here, we report mimic-enzyme catalytic CL polymer dots (hemin-Pdots) consisting of hemin and fluorescent conjugated polymer based on chemiluminescence resonance energy transfer. Hemin-Pdots show about 700× enhanced CL and over 10 h light emission in the presence of CL substrates and HO. These properties are mainly due to high-catalytic activity of hemin-Pdots and slow-diffusion-controlled heterogeneous reaction. Hemin-Pdots also possess excellent biocompatibility, good stability, emission wavelength redshift, and ultrasensitive response to reactive oxygen species (ROS), and they were successfully used for real-time imaging ROS levels in the peritoneal cavity and normal and tumor tissues of mice. Hemin-Pdots as new CL probes have wide applications in bioassays, bioimaging, and photodynamic therapy.