To develop high-performance magneto-optical chalcogenide glasses and clarify the mechanisms of the Verdet constant, a series of GeS2-Sb2S3-PbI2 chalcogenide glasses were designed and fabricated, and their Faraday effects were investigated at a wavelength of 980 nm. A new parameter, that is, average polarizability, was proposed, and the results show that the Verdet constant has a good linear relationship with average polarizability, meaning that the Verdet constant of a chalcogenide glass can be directly estimated by its chemical constituents. The Verdet constant is as large as 0.200 min G(-1) cm(-1) at 980 nm for 22.5GeS(2)-67.5Sb(2)S(3)-10PbI(2) composition glass, which is the largest value reported thus far for sulfide glasses; this glass also possesses good thermal and optical properties and therefore might be an attractive candidate for mid-infrared magneto-optical device applications.