A signal-on and sandwich electrochemiluminescence immunoassay (ECLIA) for diethylstilbestrol (DES) using magnetic surface molecular imprinting polymers (MMIPs) -antibody labeled CdS quantum dots (CdS-Ab) composite probes. Firstly, the MMIPs were synthesized through employing phenol as mimicking template to imprint inside the poly-dopamine film on the surface of Fe₃O₄@SiO₂ (MMIPs) then elute the template by 0.01M HCl. Secondly, the antibody of DES was labeled on the surface of the CdS QDs to form the signal tag (CdS-Ab).When the target, MMIPs and CdS-Ab was mixed together, a sandwich MMIPs-target-CdS-Ab composite was constructed. It can easily absorbed on the surface of a screen printed carbon electrode by external magnetic field and emit electrochemical luminescence signal at potential -0.2V. The intensity depended linearly on the logarithm of DES concentration from 0.1 to 1.0 ×10⁵ pg mL^-1, with the detection limit (LOD) of 0.03 pg mL^-1 (S/N = 3). Several samples of fish samples were tested based on the immunoassay which showed higher selectivity and good recoveries between 80% to 104%.The method was used to detect DES in fish with consistent results as that of conventional ELISA.