Abstract: The deformation of cement sheath is often inconsistent with that of casing and formation, which leads to the failures of zonal isolation. In order to solve the problems, a kind of styrene butadiene rubber latex possessing good stability was synthesized by grafting 2-acrylamido-2-methyl-propane sulfonic acid and styrene to the liquid polybutadiene with low molecular weight via seed-emulsion polymerization. The structure and properties characteristics indicate that TEL-100L latex possesses good thermal stability and perfect core-shell structure. For oil well cement stone, the evaluations of mechanical property show that TEL-100L latex can significantly reduce the brittleness coefficient and elasticity modulus of cement stone, and improve its toughness and deformation ability. The exploration of toughening mechanism finds that TEL-100L latex can not only be adsorbed onto the cement grains surface by electrostatic attraction, but also can be cross connected with cement hydration system through the complexation with Ca²⁺, which leads to the formation of compact three-dimensional net structure and the improvement of toughness and deformation ability.