Astragalusspecies have a certain capacity to enrich selenium (Se) and are the strongest Se hyperaccumulator legumes known globally at present. The biochar application to medicinal plants has been reported to affect plant metabolites. In this study, we aimed to employ hyperaccumulatingAstragalusspecies in the plant growth of selenium-lacked soil, while also investigating the impact of varying selenium doses and biochar application on legumes growth, selenium content, and secondary metabolite production. Applying biochar to soil, along with a Se concentration of 6 mg/kg, significantly enhanced the growth, Se content, total polysaccharide content, and calycosin-7-glucoside content ofAstragalusspecies (p< 0.05). Importantly, the Se and biochar application also led to a significant improvement in Se content in ABH roots (p< 0.05). Meanwhile, the content of total flavonoids in ABH roots could be promoted by a Se concentration of 3 mg/kg and biochar application in soil. Additionally, the Se enrichment coefficients ofAstragalusspecies under Se treatments were significantly higher than those under control treatment, with a marked difference observed across all treatments, whether roots or above-ground (p< 0.05). Remarkably, the Se transport coefficients ofAstragalusspecies were observed to be lower than one, except for the transport coefficient of AB in the Se concentration of the control treatment (0 mg/kg). This result showed that a medium concentration treatment of Se and biochar application in soil not only promotes the growth ofAstragalusspecies and the uptake of exogenous Se but also increases the active component content, meanwhile enhancing the Se enrichment and transport capacity. Taken as a whole, the present findings offer a more comprehensive understanding of the interplay between distinct Se levels, as well as the addition of biochar in soil, providing valuable insight for the cultivation of Se-richAstragalusin Se-deficient soil-plant systems.