Abstract: Pseudomonas sp. K3 was identified with a high ability for solubilizing tricalcium phosphate (TCP) and enhances the growth of maize seedling. This study investigated the mechanism of phosphate solubilization and assesses the influence of buffer capacity under in vitro condition on the phosphate solubilizing ability of K3. We collected filtrates from 7 days incubation in a liquid medium NBRIP, and found that the P concentration in the solution increased from 6.54 μg/mL to 655.23 μg/mL, when the pH decreased from 7.00 to 3.99. Results form HPLC-analysis showed that the solubilization of TCP was mainly caused by the release of 47.39 mmol/L malic acid, 25.67 mmol/L lactic acid and 1.89 mmol/L oxalic acid. At these concentration, malic acid, lactic acid and oxalic acid could solubilizing 535.31μg/mL, 279.06 μg/mL and 46.58 μg/mL P from TCP, respectively. It demonstrated that organic acid excretion by K3 was the main mechanism for P solubilization. Liquid culture and soil incubation experiments showed that the buffering capacity of soil could limit the solubilization of insoluble phosphate by K3 strain.