The invention discloses a method for quantitatively analyzing the content of free ion in a soil sediment sample. The method includes: dissolving and extracting the free ion from the soil sediment sample; preparing an ionic imprinting material; preparing a nonionic imprinting material; performing solid-phase extraction and analytical tests. The method has the advantages that the ionic imprinting high-polymer material is used to perform solid-phase extraction on free ion extract to eliminate the interference of a large amount of sodium ions, the method is small in error and high in accuracy, the solid-phase extraction technology can enrich, separate and purify the sample to eliminate the interference, and a new approach is provided for the accurate testing of the free iron.