In this work, the effect of the substituent at the α or β position in dialkylphosphinic acids on the extraction of trivalent lanthanide ions was investigated using experimental and theoretical methods. Five dialkylphosphinic acids were synthesized. Their extractions of seven trivalent lanthanide ions, La/Pr, Nd/Dy, and Ho/Yb/Lu, which represent light, middle, and heavy rare-earth metals, were explored. A simplified coordination model was built to explore the bonding between the extractants and lanthanide ions. The results obtained using this model generally explained the experimental results. The model could therefore be used to study the effects of the α and β substituents of extractants on their extraction properties. The theoretical and experimental results indicate that the significance of a β substituent in the dialkylphosphinic acid on extraction was greater than that of an α substituent.