The paper presents a novel method for the flotation separation of Cd2+ with ternary association complex system prior to the determination by spectrophotometry. The effects of different parameters,such as the dosages of KI and octadecyl trimethyl ammonium chloride (OTMAC),various salts and acidity etc. on the flotation yield of Cd 2+ have been investigated to select the experimental conditions. The possible flotation mechanism of Cd2+ was discussed.The results showed that by controlling pH2.0,in the presence of 1.0g (NH4)2SO4, when the dosage of 0.1 mol•L-1 KI solution was 2.00 mL and 0.01 mol•L-1 OTMAC solution was 2.00 mL respectively, the water-insoluble ternary association complex of (OTMAC)2(CdI4) which produced by Cd 2+ and I-,OTMAC cation (OTMAC+) floated above water phase and liquid-solid phases were formed with clear interface,while Mn2+ , Ni2+ ,Co2+ , Zn2+ and Al 3+ could not be floated,so Cd2+ was floated quantitatively at pH2.0. Thereby, the quantitative separation of Cd2+ from the above metal ions could be achieved.A new method of determination of trace cadmium by flotation separation was established.The proposed method has been successfully applied to the determination of Cd 2+ in the sample of synthetic water and the flotation yield was 97.2%~108.7%.