[Objective] As soil acidification in southwestern China becomes increasingly severe, the labile phosphorus pool is transformed into a non-labile phosphorus pool, which reduces the availability of soil phosphorus, affecting crop yield and wasting phosphate fertilizer resources. In this study, we prepared a biochar-immobilized phosphorus-solubilizing bacterial agent with biochar as the carrier and a strain capable solubilizing both organic phosphorus and inorganic phosphorus as the immobilized strain and then optimized the preparation conditions. Furthermore, this bacterial agent was evaluated in terms of the stability and the solubilizing effects on insoluble phosphorus. [Methods] Selective media were used for the isolation of phosphorus-solubilizing bacteria from plant rhizosphere soil. The molybdenum-antimony colorimetric method was employed to quantify the ability of bacteria to solubilize phosphorus. The bacterial strain was identified through physiological and biochemical tests and molecular biological analysis. The immobilized bacterial agent was prepared by the adsorption method, and the preparation conditions were optimized by single factor experiments. The prepared agent was characterized by Fourier transform infrared spectrometry and scanning electron microscopy. Furthermore, the metabolic spectrum of organic acids and phosphatase activity were qualitatively and quantitatively tested by HPLC and the fluorescence method, respectively. [Results] The strain Klebsiella sp. was isolated for immobilization, and its abilities to solubilize lecithin and tricalcium phosphate were 236.5 mg/L and 200.3 mg/L, respectively. Genome analysis showed that the strain N107 carried 27 genes related to organic and inorganic phosphorus solubilization. The optimized preparation conditions were biochar addition of 30.0 mg/mL, N107 inoculation amount of 6.0%, immobilization temperature of 30.0 ℃, and immobilization time of 12.0 h. The bacterial agent prepared under the optimal conditions increased the phosphorus-solubilizing capacity for lecithin and tricalcium phosphate by 24.0% and 22.5%, respectively, compared with the free bacterial strain. The biochar-immobilized phosphorus-solubilizing bacterial agent contained more oxygen-containing functional groups, compared with the original biochar, its total specific surface area and external surface area increased by 61.9% and 165.1%, respectively. The mechanism of phosphorus solubilization by the immobilized bacterial agent was preliminarily analyzed. The results showed that the levels of tartaric acid, citric acid, and total acids changed significantly and the activities of acid and alkaline phosphatases in the culture medium were effectively improved, although the types of organic acids secreted by the agent had no obvious changes. The structural equation model showed that pH value was closely related to phosphatase activity and organic acid content, and the immobilized bacterial agent can promote the activation of insoluble phosphorus by increasing phosphatase activity and organic acid content. [Conclusion] The immobilized phosphorus-solubilizing bacterial agent prepared in this study provides a good bioremediation material for the activation of insoluble phosphorus. This study provides an innovative perspective for developing green remediation strategies based on microbiomes.