Abstract: In this paper,water-oxalic acid-TOA(octanol) is used as the experimental system,and TOA,octanal as complexing agent,diluent respectively.The extraction mechanism of oxalic acid by TOA and the influence of the concentration of TOA ( S₀) and solute on extraction equilibrium are studied.The structure of complex in organic phase is measured by infrared spectroscopy,and it is obtained that there are two types of complexates in organic phase,1∶1 and 1∶2 (acid∶TOA).The distribution coefficient increases with S₀.Loading of complexing agent shows an increasing tendency with S₀ and the equilibrium concentration of acid in aqueous phase( x ).The saturated loading is achieved at higher x and increases with S₀.The methematical model with two type complexes is proposed based.The model parameters, K₁₁ and K₁₂ are obtained by fitting the experimental data of extraction equilibrium,and K₁₁ is much larger than K₁₂ .