The aim of this study was to adsorb Ca, K, Mg and Na from seawater using sulfonated cornstalk. Cornstalk, a ligno-cellulose derived agricultural waste, was selected as an adsorbent. It was separated into the inside and outside and then sulfonated to be used as an adsorbent. Sulfonated cornstalk has higher ion exchange capacity than natural cornstalk, and the largest ion exchange capacity was 4.35 meq/g of sulfonated outside cornstalk. In addition, according to the FT-IR analysis, the surface of cornstalk was converted to negative charge after sulfonation, so that it was very easy to adsorb cationic ions in seawater. The adsorption efficiency of the sulfonated cornstalk increased by 27.96-41.58% for Ca, 22.61-26.78% for K, 27.96-44.14% for Mg and 18.47-27.16% for Na compared to the non-sulfonated cornstalk. Surface modified cornstalk with sulfuric acid can efficiently adsorb Ca, K, Mg and Na from seawater. This is meaningful not only in terms of recycling ligno-cellulose based waste, but also adsorbing useful resources from seawater.