###### Abstract  
In order to study the hydration absorption behaviors and characteristics of sandstone in Mogao Grottoes in China, the pressure-less hydration absorption experiment on the outcrop porous sandstone of Mogao Grottoes was carried out by using the self-developed real-time monitoring experimental system. The hydration absorption was measured and the curve of hydration absorption with time was drawn. At the same time, the digital image correlation method (DIC) was used to measure the full-field deformation, and the speckle pattern of the sample was analyzed using Match ID, and the displacement field and strain field of the sandstone sample at different hydration absorption moments were computed. Moreover, the sparse area and dense area of sandstone are used as regions of interest (ROI) for DIC analysis. According to the test results, it is concluded that the hydration absorption of sandstone increases rapidly in the initial stage, and gradually tends to be stable with the change of time. This corresponds well with the deformation characteristics of sandstone analyzed using DIC. In the initial stage, the deformation of sandstone increases rapidly. With the change in time, the deformation of sandstone samples gradually slows down. When the hydration adsorption reaches saturation, the sandstone continues to deform for a period of time before stopping hydration absorption. The results of the mercury injection test and the XRD test show that the porosity of the sparse area is larger than that of the dense area and the particle content of the dense area is lower. When the sandstone is saturated with water, the liquid is immersed in the pores between the solid particles, which makes the sparse area more prone to stress concentration, and the deformation in the sparse area is larger. Therefore, when analyzing the hydration absorption deformation of sandstone, the porosity should be considered. hydration absorption detection; displacement field; outcrop porous sandstone; rock deformation; digital image correlation method (DIC)  
The hydration absorption of rock has been conducting some mechanical behaviors and mechanism researches. [PERSON] et al. [3] have carried out macroscopic and microscopic studies on the water swelling characteristics and deformation mechanism of rocks, and carried out multi-scale studies on the physical and mechanical properties of rocks, and reached certain conclusions. [PERSON] et al. [4] studied the physical and mechanical characteristics of sandstone under the dry-wet cycle, and concluded that the change of mineral composition and microstructure is the main reason for the deterioration of sandstone strength. [PERSON] et al. [5] used an indoor uniaxial compression test to study the coupling law of water swelling and creep of mudstone under different initial water content, water gushing mode, and loading mode. It is concluded that the creep deformation increases with the increase in hydration absorption area. [PERSON] et al. [6] used nuclear magnetic resonance (NMR) technology to measure the porosity and pore size distribution of sandstone samples in different chemical solutions. It is concluded that the porosity of sandstone increases after hydration and absorption of chemical solution, and the weakening law of compressive and tensile strength of sandstone under static and dynamic conditions is similar, which is exponentially related to the damage variable. [PERSON] et al. [7,8] studied the strength softening and microscopic mechanism of sandstone strata under different water content or temperature conditions. [PERSON] et al. [9] conducted dynamic uniaxial compression tests on sandstone specimens after dry-wet cycle treatment under the same loading conditions, and studied the influence of water temperature and dry-wet cycle coupling on the dynamic mechanical properties of sandstone.