In order to study the fluorescent dye thiazole orange as a fluorescent molecular switch and a cadmium ion nucleic acid aptamer for the detection of cadmium ions. The effects were examined, from different pH values, thiazole orange-aptamer concentration ratio, binding time of thiazole orange to aptamer, and binding time of aptamer to cadmium ion on the fluorescence intensity of the system. The results showed that when the pH value was 7, the thiazole orange-aptamer concentration ratio was 7∶3, the binding time of thiazole orange and aptamer was 15 min, and the binding time of aptamer and cadmium ion was 20 min. Under these conditions, the fluorescence intensity and cadmium ion concentration showed a good linear relationship, with a linear range from 2.00 ng/mL to 80.00 ng/mL, and the detection limit was 0.23 ng/mL. This method does not require fluorescent labeling of the aptamer, and the operation is simple.