A method for a cation-exchange resin wherein a strongly acidic cation-exchange resin is contacted with an aqueous eluting solution and the polystyrenesulfonic acid being eluted from this resin is measured, which comprises setting a plurality of molecular weight ranges in the molecular weight distribution of the polystyrenesulfonic acid eluted, and evaluating the performance capability of the cation-exchange resin based on the correspondence relationship of each molecular weight range with the amount eluted in said each molecular weight range. The evaluation method allows on-target and precise evaluation of the performance capability of a cation-exchange resin being independent of the structure of a base resin and the circumstance under which it is used, which leads to the determination of the optimum time for the exchange of a resin in a water treatment system using a cation-exchange resin, and thus to the extension of exchange of a resin and the reduction of an operation cost for the system.