In order to quantify the speciation and structures of silver-sulfide complexes in aqueous solutions, we have carried out systematic first-principles molecular dynamics (FPMD) simulations at three temperatures (25, 200, and 300 degrees C). It is found that monosulfide (i.e., Ag(HS)) and disulfide species (i.e., Ag(HS)(2)(-)) are the major silver-sulfide species over a wide T-P range, while Ag(HS)(3)(2-) can hold stably only at ambient temperatures, and Ag(HS)(4)(3-) does not exist even at the ambient conditions. Ag(H2S)(+) has a tetrahedral structure up to 300 degrees C (i.e., Ag(H2S)(H2O)(3)(+)). Ag(H2S)(2)(+) remains 4-coordinated to 200 degrees C (i.e., Ag(H2S)(2)(H2O)(2)(+)), but it transforms to 3-coordinated at 300 degrees C (i.e., Ag(H2S)(2)(H2O)(+)). All of the other mono and disulfide species (Ag(HS)(H2O)(0), Ag(HS)(OH)(-), Ag(HS)(H2S)(0), Ag(HS)(2)(-), and AgS(HS)(2)(-)) have 2-fold linear structures. For their solvation structures, the H2S ligands donate weak H-bonds to water O; the HS- ligands accept weak H-bonds from water H; the dangling S2- form strong H-bonds with H of water molecules, and the OH- ligands can form strong H-bonds as donors and weak H-bonds as acceptors. We further calculated the acidity constants (i.e., pK(a)s) of Ag(H2S)(+) and Ag(H2S)(2)(+) complexes using FPMD based vertical energy gap method. Based on the calculated pK(a)s, the mono- and disulfide species distributions versus pH have been derived. We found that for monosulfide species, Ag(HS)(H2O)(0), is the major species in near neutral pH, while Ag(H2S)(H2O)(3)(+) and Ag(HS)(OH)(-) exist in the acid and alkaline pH range at T <= 200 degrees C, respectively. At 300 degrees C, both Ag(HS)(OH)(-) and Ag(HS)(H2O)(0) are dominant in the neutral pH range, and Ag(H2S)(H2O)(2)(+) only exists in acidic solutions. For disulfide species, Ag(HS)(2)(-) is dominative in near neutral pH condition at the three temperatures; Ag(HS)(H2S)(0) stays in mild acidic pH range only at 25 degrees C; AgS(HS)(2-) and Ag(H2S)(2)(H2O)(2)(+) (Ag(H2S)(2)(H2O)(+) at 300 degrees C) are trivial at the three conditions. The results of structures and acidity constants provide quantitative and microscopic basis for understanding the behavior of silver complexes in hydrothermal fluids.