Selenium (Se) can enhance stress resistance and alleviate the abiotic stress in plants. This study investigates the effect of Se on cadmium (Cd) uptake by the Cd-zinc hyperaccumulator Sedum plumbizincicola, aiming to provide a scientific basis for using Se to regulate Cd uptake and accumulation in Sedum plumbizincicola. Here, a hydroponic experiment was conducted with two Cd concentrations and four Se concentrations, resulting in eight treatments. The growth, physiological parameters, and Cd uptake indicators of S. plumbizincicola were measured under different treatments. The results showed that under the 5 μM Cd treatment, the application of different Se concentrations promoted the growth of S. plumbizincicola, increased chlorophyll content and antioxidant enzyme activity, and increased the Cd uptake in roots, but reduced Cd accumulation in shoots. Under the 100 μM Cd treatment, exogenous Se application had no significant effect on plant biomass and chlorophyll content, but still increased antioxidant enzyme activity and Cd uptake in roots. There was no significant difference in shoot Cd uptake of S. plumbizincicola under different Se treatments. Under different Cd concentrations, Se application increased the proportion of the soluble Cd fraction but decreased Cd percentage bound in the cell wall component in leaves and stems. Similarly, Se addition promoted the Cd speciation transformation from the more active ethanol- and deionized water-extractable forms to the more stable acetic acid- and hydrochloric acid-extractable forms, with this effect being significantly more pronounced under low Cd than under high Cd treatment. This study suggests that when the added Se/Cd molar ratio in the hydroponic solution is less than 1, it can promote the growth of S. plumbizincicola to a certain extent without significantly affecting shoot Cd accumulation. When the added Se/Cd molar ratio in the hydroponic solution is greater than or equal to 1, it reduces shoot Cd accumulation in S. plumbizincicola, but does not change its hyperaccumulation characteristics with bioconcentration factor and translocation factor remaining greater than 1.