The present invention relates to a resin composition and a manufacturing method, which are capable of obtaining a thiourethane-based optical material by reacting a polythiol compound having three or more mercapto groups and a polyisocyanate compound, and, specifically, to a resin composition and a manufacturing method, which are capable of manufacturing a high-quality ultrahigh-refractive-index thiourethane-based optical material by controlling a polymerization rate through an additive while reducing an effect according to the quality of a polythiol compound so as to lower manufacturing costs. The present invention provides a resin composition for a thiourethane-based optical material, comprising: (a) a polythiol compound having three or more mercapto groups; (b) a polyisocyanate compound; and (c) a catechol compound represented by chemical formula 1, wherein the catechol compound is included in an amount of 0.05-5 wt% on the basis of 100 wt% of the composition, the amount being the sum of (a), (b) and (c). The present invention can manufacture a high-quality ultrahigh-refractive-index thiourethane-based optical material while simplifying a process for producing a polythiol compound that has three or more mercapto groups and reducing production costs, and thus can be widely used in the optical lens field.