A method of preparing an optical material by polymerizing a resin composition including a thiol group-containing compound and an isocyanate group-containing compound, particularly, a method of preparing a high-quality urethane-based optical material using a universal polyisocyanate compound. According to one aspect, a resin composition including a polythiol compound and a widely available polyisocyanate compound, as main components, and having a moisture content of 300 to 3,000 ppm is template-polymerized to prepare the optical material. A colorless, transparent, and high-quality urethane-based optical material may be efficiently prepared in high yield using a low-cost and widely available isocyanate compound through template-polymerization using a tape while minimizing generation of white tape residues and foaming.