Influence of styrene-butadiene emulsion on gypsum was studied. Results show that styrene-butadiene emulsion can reduce required content of mixing water, accelerate initial and final setting and increase bulk density of gypsum. Saturated strength of modified hardened gypsum is effectively enhanced. Water absorption and dissolution of modified hardened gypsum decrease greatly. Thus compressive and flexural strength and water resistance are improved a lot. Films of styrene-butadiene copolymer begin to form after dehydration. Crystals of gypsum are wrapped by films and thus porosity of modified hardened gypsum decreases.