The function principle of the cation exchange resin is to use its own functional group to react with the positive ions in the water, adsorb the ions in the water to its own surface, and achieve the purpose of purification from the surface.

Cation exchange resin mainly contains acidic groups in solution, such as sulfonic group (- SO₃H), carboxylic group (- COOH) and phenolic group (- C₆H ₄OH), in which hydrogen ion can exchange with metal ion or other cations. For example, a polymer of styrene and divinylbenzene is sulfonated to obtain a strong acidic cation exchange resin. Its structural formula can be easily expressed as R-SO₃H, R represents resin matrix, and the exchange principle is 2R-SO₃H+Ca2+- (R-SO₃) ₂Ca+2H+], which is also the principle of hard water softening.

The function of ion exchange resin is in the solution of natural environment, the positive ions (Na+, Ca2+,? K+,? Mg2+, Fe3+, etc.) and cation exchange resins (sulfonic group (- SO3H), carboxylic group (- COOH) or phenolic group (- C₆H₄OH) and other acid-base groups of metal materials. It is easy to generate H+ionization in water). The ion exchange method promotes the transfer of positive ions from aqueous solution to cation exchange resin. The H+exchange of cation exchange resin is water (that is, the basic principle of cation exchange resin).

The cations (Cl -, HCO₃ -, etc.) in the solution exchange with the OH - on the cation exchange resin (with quaternary amine [- N (CH₃) ₃OH], amine (- NH₂) or imino (- NH₂) and other meta-base groups, which are easy to generate OH - ionization in water). The cations in the water are transferred to the epoxy resin, and the OH - on the resin is exchanged to the water, (that is, the basic principle of cation exchange resin). However, H+and OH - are closely combined and converted into water, thus exceeding the target of desalting.