Objective To design a novel impedimetric immunosensor based on AgI mimic enzyme nanomaterial for detecting carcinoembryonic antigen (CEA) in serum with high sensitivity. Methods A novel chitosan modified AgI (CS-AgI) nanomaterial was synthesized and was characterized by transmission electron microscope (TEM) and Fourier transform infrared spectroscopy (FTIR). Then by utilizing CS-AgI labeled CEA antibody as tags, we prepared a novel impedimetric immunosensor on the gold electrode using the sandwich-type immunoassay. The electrochemical properties of the prepared impedimetric immunosensor were observed by electrochemical workstation and the concentration of CEA in sample was quantitatively analyzed. Results The synthesized CS-AgI nanoparticles were spherical in shape, with the particle size being 100 nm; the particles were employed to construct immunosensor as signal markers. The immunosensor had an excellent electrochemical performance in detection of CEA under PBS base solution of pH=7, and its AC impedance response increased with the increase of the logarithm of CEA concentration, exhibiting a good linear relationship in the range of 0.1 ng/mL to 80 ng/mL (r=0.996), with a detection limit of 0.05 ng/mL. Conclusion Based on AgI mimic enzyme nanomaterial, the impedance immunosensor prepared in this study shows a high sensitivity in detecting CEA; meanwhile, it has acceptable selectivity, repeatability and stability, providing an experimental evidence for early diagnosis and treatment of cancer.