The invention relates to an adsorbent and a preparation method thereof and in particular relates to an adsorbent for adsorbing small molecular gas and a preparation method thereof. The adsorbent comprises a physical adsorbent and chemical adsorbents, wherein the chemical adsorbent is attached on the physical adsorbent. The adsorbent takes the conventional physical adsorbent for treating organic waste gas as a carrier, and the chemical adsorbents including magnesium chloride, calcium chloride, copper sulfate and the like are attached on the physical adsorbent; a working principle is similar with that of the conventional physical adsorbent; the adsorbent has good technology compatibility and an efficient adsorption capability on low-carbon-number small molecules including methanol, ethanol and the like, so that the treatment effect on the organic waste gas is greatly improved; and the preparation method of the adsorbent is simple and economical and is suitable for large-scale industrialpopularization and application.