Abstract: Metal ions have a very important influence on the flotation process of minerals, and some of them can activate minerals and thus yield an improvement in the flotation effect. Some production practices have shown that Pb²⁺ has an activation effect in the process of cassiterite flotation, which can improve the rate of cassiterite recovery. Styrene phosphonic acid is the most commonly used collector in the production of cassiterite flotation. In this study, the activation effect of Pb²⁺ in cassiterite flotation when styrene phosphonic acid is used as a collector is revealed by single mineral flotation tests. The activation mechanism of Pb²⁺ in the process of styrene phosphonic acid collecting cassiterite is assessed by contact angle measurement, zeta potential determination, IR spectroscopy, and solution chemistry analysis. The results of single mineral flotation tests indicate that Pb²⁺ can increase the floatability of cassiterite when the pH is 2.0~8.0, and at a pH of 4.0, the recovery rate of cassiterite reaches maximum, 93.78%, which is 5.33% higher than the recovery rate without Pb²⁺. The results of zeta potential determination, IR spectroscopy, and solution chemistry analysis show that the styrene phosphonic acid can be adsorbed on the cassiterite surface in form of chemical adsorption, causing the zeta potential of the surface to shift toward the negative direction, and the Pb²⁺ can promote the adsorption of styrene phosphonic acid on the cassiterite surface, making the zeta potential of the surface lower. Moreover, the Sn⁴⁺ on the cassiterite surface can be replaced with Pb²⁺ and the hydrolyzed species PbOH⁺ in solutions can interact with Sn-OH on the surface to form the surface complex Sn-O-Pb⁺, which may lead to an increase in the number of active sites on the cassiterite surface, promoting the adsorption of styrene phosphonic acid on the cassiterite surface and resulting in the activation of cassiterite.