The removal of arsenate in water is closely related to its hydration, but there are few reports on the hydration characteristics of different protonated arsenates and iron arsenates, and there is no correlation analysis on infrared spectra of hydration layers of protonated arsenates and iron arsenates. The hydration energies of different protonated arsenates [H_mAsO₄]^m-3(m=0~2) and iron arsenates [FeH_mAsO₄]^m+(m=0~2) were compared at B3LYP/6-311G(d, p) level. Reduced density gradient functions conducted graphical analyses for the intensities, types and locations of the interaction between water molecules with [H_mAsO₄]^m-3(m=0~2) and [FeH_mAsO₄]^m+(m=0~2). And, the characteristics of infrared spectra of the hydration layers of different protonated arsenates and iron arsenates were analyzed. The results show that the hydration of [H_mAsO₄]^m-3(m=0~2) gradually decreases with hydrogen protonation, while the protonation enhances the hydration of [FeH_mAsO₄]^m+(m=0~2). Hydrogen bonds tend to form when a water molecule hydrogen interacts with an oxygen of [H_mAsO₄]^m-3(m=0~2). However, when two hydrogens of water molecules simultaneously interact with two oxygens of [H_mAsO₄]^m-3(m=0~2), the interaction becomes weaker, and the van der Waals force appears. The hydrogen bond formed by water molecules through hydrogen with the oxygen of arsenates is stronger than the hydrogen bond formed by water molecules through oxygen with the hydrogen of protonated arsenates. The unprotonated O_N tends to form hydrogen bonds with 2~4 water molecules, while the protonated O_P forms hydrogen bonds with at most 2 water molecules, and the O_P…H_W hydrogen bond is weaker than the O_N…H_W hydrogen bond. In the infrared spectra, 2 954, 3 114, 3 179, 3 252 and 3 297 cm^-1 is the stretching vibration peaks of Ow—Hw in the first hydration shell of As₄^3-; 3 277, 3 324 and 3 376 cm^-1 is the stretching vibration peaks of Ow—Hw in the first hydration shell of HAs₄^2-; 3 189, 3 277, 3 306 and 3 383 cm^-1 is the stretching vibration peaks of Ow—Hw in the first hydration shell of H₂As₄^-. The stretching vibration regions for Ow—Hw in the first hydration shell of [FeH_mAsO₄]^m+(m=0~2) are 2 500~3 060, 2 660~3 200, 2 900~3 360 cm^-1. Therefore, the stretching vibration regions for the first hydration waters of [H_mAsO₄]^m-3(m=0~2) and [FeH_mAsO₄]^m+(m=0~2) have blue shifts with protonation. Compared with [H_mAsO₄]^m-3(m=0~2), the water molecules in the first hydration layers of [FeH_mAsO₄]^m+(m=0~2) exhibit a significantly red shift of the bending vibration peaks and stretching vibration peaks in the infrared spectra. The hydrogen bond bridge Fe—Ow—Hw…Ow—Hw…O_N—As is formed in the first hydration shell of [FeH_mAsO₄]^m+(m=0~2). The Ow—Hw in this hydrogen bond bridge has a special absorption peak, such as the stretching vibration peak located in 2 195, 2 526 and 2 673 cm^-1, respectively. Its stretching vibration peak is significantly blue-shifted, but the peak strength is almost unchanged, while its bending vibration peak is red-shifted with the protonation and the strength is significantly reduced. The stretching vibration peak of independent O_P—H is not affected by the complexation of Fe, while the position of stretching vibration peak for O_P—H in O_P—H…Ow is significantly blue shifted, due to the complexation of Fe. This study is helpful to understand better the solubility of arsenates and iron arsenates in water at different pH.