The development of simple and ultrasensitive immunoassays is critical for the early diagnosis and treatment of diseases. The efficient integration of amplification strategies with highly sensitive detection probes plays a key role in boosting the ultrasensitive immunoassays. In this paper, we report a multifunctional and integrated dopamine-modified conjugated polymer nanoparticle (DA-CPN) probe prepared by a one-step nanoprecipitation method for ultrasensitive immunoassay. The multifunctional DA-CPNs fully integrate the unique properties of dopamine and fluorescent conjugated polymer nanoparticles, while possessing three key capabilities. 1, DA-CPNs can be rapidly deposited onto adjacent proteins catalysed by horseradish peroxidase (HRP) labelled in the detection antibody in a sandwich immunoassay. 2, DA-CPNs undergo self-polymerization simultaneously, resulting in the assembly of large numbers of CPNs and thereby amplifying the detection signal. 3, CPNs possess excellent fluorescence brightness and strong photobleaching resistance, further enhancing the sensitivity of fluorescence immunoassays while simplifying the experimental procedure. Using carcinoma embryonic antigen (CEA) as a model, the proposed method demonstrated a wide linear range spanning five orders of magnitude and an exceptional sensitivity with a detection limit of 0.39 fg/mL. Therefore, this study based on DA-CPNs provides a versatile and highly promising platform for the ultrasensitive immunoassays and in vitro diagnosis of diseases.