Abstract: The heavy metal Cd²⁺ has become one of the major risks to human health and environmental safety due to its characteristics of extreme toxicity and wide distribution. Therefore, it is particularly important to establish sensitive, rapid and reliable detection methods. An aptasensor based on fluorescence resonance energy transfer was designed to detect Cd²⁺ using carboxyfluorescein (FAM) and black hole quencher (BHQ1) as the fluorescence energy donor and acceptor pairs. In the absence of Cd²⁺, Aptamer (Apt) hybridized with its complementary chain to form a double-stranded structure. The distance between FAM and BHQ1 modified at the end of the double chain was shortened, which induced the fluorescence resonance energy transfer effect. Finally, the fluorescence of FAM was quenched and the fluorescence intensity of the sensor was significantly reduced. When Apt, cDNA and Cd²⁺ were introduced in the sensing system at the same time, Cd²⁺ will preferentially bind to the aptamer specifically to form the Cd²⁺-Apt complex with a special structure. It prevented the formation of double chain to protect the fluorescence of FAM from quenching. Then, the fluorescence intensity of the sensor was recovered. Thus, the quantitative analysis of Cd²⁺ could be realized by the change of fluorescence intensity of FAM. The experimental results showed that the optimal binding mode was the simultaneous incubation of Apt, cDNA and Cd²⁺. The best complementary chain was cDNA2, which complemented the Apt by 20 bases. The optimal concentration ratio between Apt and cDNA was 1:1. The optimum incubation time was 30 min. The optimum incubation temperature was 37℃. Under the optimal conditions, the prepared fluorescence aptasensor showed a linear response to Cd²⁺ in the range of 5 to 5 000 nmol/L. The limit of detection (LOD) of Cd²⁺ was 0.94 nmol/L. The recoveries of tap water, green tea, and grains ranged from 90.3% to 110.7%. Therefore, the prepared fluorescent aptasensor in this study is effective in detecting Cd²⁺ in food samples.