In the conditions of pH >1, 0<pH≤1 and pH<0, P ₂₀₄-kerosene system and the organic phase loading La in the P ₂₀₄-HCl-H ₃AOH were studied by infrared spectroscopy. The effect of acidity on extraction mechanism of La(Ⅲ) in P ₂₀₄-HCl-H ₃AOH extraction system was investigated. The results show that displacement reaction between La and P-OH, and the mechanism is cation exchange when the value of pH>1. Both the group of P=O and the group of P-OH react with La and the extraction mechanism is the dual mechanism theory of cation exchange mechanism and salvation complex mechanism. Thus, the extraction capacity of La in the condition of 0<pH≤1 is increased. When the value of pH≤0, the two groups of P=O and P-OH, especially P=O, take part in the complex reaction.