Calcium polysulfide (CaS5) is widely used in stabilizing Cr(VI)-contaminated soils. The stability properties of Cr(VI) and Cr are the most important indexes for evaluating the effectiveness of stabilized soil. This study investigated the effect of CaS5 dosage and curing age on the stability properties of Cr(VI) and Cr in contaminated soils and the relationship between leachability and Cr speciation. Results show that increasing the CaS5 dosage and curing age could improve the stability properties of Cr in stabilized soil. The leachability and Cr(VI) content in the stabilized soil significantly decreased along with increasing CaS5 dosage and curing age. The changes in the leachability of the soil were attributed to the changes in the Cr speciation distribution and microstructure of the stabilized soil. The exchangeable fraction was mainly converted into an oxidizable fraction, and a dense structure (ettringite and elemental sulphur) was formed along with increasing CaS5 dosage and curing age. The exchangeable and reducible fractions depended on Cr leachability, and the Cr in the synthetic precipitation leaching procedures predominantly resulted from the exchangeable and reducible fractions.