In-lab simulation of low molecular weight organic acids, i.e. oxalic acid, citric acid, tartaric acid and malic acid, activating inorganic phosphorus (P) in red soil was performed. When the same in concentration, organic acids followed an order of citric acid > tartaric acid > malic acid in capability of activating soil phosphorus. When their concentrations were < 0.5 mmol L^-1, oxalic acid was the lowest and when ≥ 5 mmol L^-1, it is still oxalic acid that was the highest in the capability. The P activating capability of the organic acids declined with rising pH. When they were the same in pH, they activated Al-P the most, Fe-P and Ca-P the next, and O-P the least. It is suggested that P release from phosphates in soils induced by organic acid should be attributed to the joint effect of acidity effect of proton and complex effect of organic anion, and closely related to solubility product constant of inorganic phosphate. The findings are of important significance to the study on cycling of inorganic P in the rhizospheric soil.