Provided is a method for producing a resin lens, comprising: A) mixing a polyisocyanate, a modified isocyanate, a catalyst and a release agent, and performing vacuum defoaming at 0˜30° C. for 10˜90 min to obtain a material a; B) mixing the material a with a sulfur-containing compound, and performing vacuum defoaming at 15˜20° C. for 20˜120 min to obtain mixed monomers; and C) completing casting of the mixed monomers, and curing to obtain a resin lens. The present disclosure uses polyisocyanate and a modified isocyanate at the same time to prepare a resin lens with higher glass transition temperature and higher surface hardness without producing bank mark and edge fogging. The present disclosure further provides a method for producing modified isocyanate. The obtained modified isocyanate used with polyisocyanate further improves the glass transition temperature and surface hardness of the resin lens without producing bank mark and edge fogging.