The present invention relates to a method for manufacturing a high-quality thiorethane-based optical material by using a universal polyisocynate compound, a resin composition therefor, and an optical material manufactured thereby. The present invention provides the thiourethane-based optical material, which is not subject to whitening of the edge of the lens or polymer imbalance, by controlling a polythiol compound obtained from the manufacturing process to be pH 3.1-7, in order to resolve the existing problem of whitening, which occurs by an adhesive agent melting from an adhesive tape when casting the lens.