A promising electrochemiluminescent immunosensor based on CdS–Fe₃O₄ nanocomposites was developed for the detection of Ochratoxin A. The electrochemiluminescent emitters CdS QDs were immobilized onto mesoporous Fe₃O₄. With the use of mesoporous Fe₃O₄ with high loading of CdS QDs and antibodies, the electrochemiluminescent immunosensor showed intense electrochemiluminescence radiation and a sensitive response signal. The method showed a wide linear range from 0.01–100 ng mL⁻¹ with a low detection limit of 2 pg mL⁻¹ (S/N = 3). The proposed electrochemiluminescent immunosensor is simple, rapid, sensitive, specific, stable and reliable. It provides a powerful avenue for the design of an ultrasensitive electrochemiluminescence detection method, showing great promise in food detection.