The transformation of mercury fractions under both natural environmental conditions and ultraviolet irradiation (UV) conditions was studied in this work. Mercury in desulfurization gypsum was divided into five fractions by sequential extraction procedure depending on its bioavailability. The five fractions were named as water soluble fraction (F1), ion-exchangeable fraction (F2), acid soluble fraction (F3), elemental fraction (F4), and sulfide fraction (F5). The results from our study demonstrated that the proportion of different fraction was in the following order: elemental fraction > water soluble fraction > acid soluble fraction > ion-exchangeable fraction > sulfide fraction. The results indicated that mercury could be released from desulfurization gypsum during aging process and the release process could be promoted via ultraviolet irradiation. The release amount increased with the irradiation time and intensity (up to 25.1% of the total initial amount). The percentage of mercury in F1 decreased gradually with aging time, while the percentage of mercury in elemental and residual fractions increased gradually. Our research will be helpful for the survey and understand of mercury emission from desulfurization gypsum.