Abstract: Objective: Explore the conditions of preconcentration and separation of Cd²⁺ in water samples by nano-SiO₂, prepare monoclonal antibody against Cd²⁺ and establish a rapid detection method of trace Cd²⁺ in water samples by heterologous indirect competitive ELISA (hicELISA). Methods: Cd²⁺ was preconcentrated by nano-SiO₂ and eluted by EDTA-2Na. The content of Cd²⁺ was determined by ICP. Balb/C mice were immunized with Cd-ITCBE-BSA to prepare Cd²⁺ monoclonal antibody. The heterologous coating antigen was synthesized to establish the detection method of cadmium ion hicELISA. Results: The adsorption capacity of Nano-SiO₂ for Cd²⁺ was 13.3 mg/g, and the enrichment multiple was 20 times. EDTA could be used as the effective eluent for the pretreatment. Using Cd-ITCBE-BSA as immunogen, the titer of serum was 1:1.28×10⁴, The titer of mAb in ascites was 1:2.0×10⁵ and the affinity constantwas 8.1×10⁸ L/mol. The detection limit of hicELISA was 2.9 μg/L. There was no significant cross reaction with Hg²⁺, Cu²⁺, Zn²⁺, Pb²⁺, Cr³⁺, Mo⁶⁺, Fe³⁺, Co²⁺. The recoveries of cadmium in lake water, tap water and ultrapure water were 92.47%~102.86%. Conclusion: The hicELISA method was sensitive, specific and accurate, and could be used for the detection of cadmium in water samples.