The interferences of dissolved oxygen towards the electrochemical immunosensors occurred constantly, which will restrict the practical clinical application of electrochemical immunoassay. Herein, a new enzyme-free strategy has been proposed for ultrasensitive electrochemical immunoassay of carcinoembryonic antigen (CEA), which can completely avoid the interference of dissolved oxygen. Porous PdCu nanoparticles can catalyze aniline to produce polyaniline by the inherited peroxidase-like activity for generating electrochemical detection signal. More interesting, with the assistance of electropolymerization strategy, the electrochemical measurement signal could enhance sharply. Accordingly, we established a sandwich-type electrochemical immunosensor based on porous PdCu nanoparticles produced polyaniline with electropolymerization strategy for enzyme-free and dissolved oxygen interference-free detection of CEA, affording a satisfied performance with a broad linear detection range (0.1 pg/mL ∼ 10 ng/mL) and low detection limit (0.08 pg/mL). This method might be considered as a robust approach for clinical applications.