###### Abstract
(2018) obtained a red sandstone pore characteristics map using the scanning electron microscopy method and extracted the key parameters for evaluating fractal permeability, such as curvature fractal dimension, pore area fractal dimension and maximum pore area, from the segmented images using the image enhancement technique. [PERSON] et al. (2018) measured the changes in the microstructure of red sandstone specimens after freeze-thaw cycles using the nuclear magnetic resonance (NMR) technique to study the change patterns of defects such as fracture surfaces and porosity. Studies on the durability and degradation of sandstones by erosion have been continuously carried out due to their high porosity, low strength and poor durability. [PERSON], [PERSON] et al. (2018) investigated the effects of freeze-thaw (F-T) and salt crystallization (S-C) phenomena on the strength and durability of sandstones in the upper red strata by testing the variation in ultrasonic velocity (V (p)), weight loss (%) and point load index. [PERSON], [PERSON] et al. (2018) studied the properties of fully weathered coastal red sandstone by predicting the shear strength of fully weathered red sandstone from in situ measured physical property parameters such as water content, fines content and relative compaction. [PERSON] et al. (2018) tested the compressive mechanical properties of different historical building sandstones in Portugal, demonstrating the central role of porosity in the compressive behavior of redundant sandstones, and developed an analytical model for predicting the compressive behavior based on the porosity of building stones. [PERSON], [PERSON] et al. (2018) used XRD and XRF methods to analyze in detail the petrography of red sandstones in the sedimentary environment of northern Kuwait, to study the variation pattern of the material contained in the sandstones in different parts. In summary, for ascertain the current physical properties, durability and damage change law of red sandstone, a large number of studies have been conducted, mainly focusing on the environmental resistance of red sandstone, such as freeze-thaw cycles, hot and cold cycles, high temperature loading and so on. These have been investigated by way of experiments to study red sandstone in different environments, its performance and the damage change law. However, there is less research on the degradation of red sandstone under natural environmental erosion, especially on the red sandstone of existing ancient buildings. To address the degradation degree and characteristics of red sandstone for existing ancient buildings, we consider Qili ancient town and use samples of red sandstone from its ancient buildings to analyze the chemical mineral composition, weathering and corrosion degree. Since sampling is also a kind of damage to ancient buildings, we obtained specimens from the ancient wharf constructed at the same time as the local ancient building complex, which is more than 800 years old, and then used nondestructive testing methods, such as ultrasound, SEM, XRD, and X-CT, to test the physicochemical properties of the red sandstone specimens. The characteristics of the degradation degree in different directions were analyzed using the characteristic parameters of the gray distribution map of the CT image, and an overall plane degradation degree was defined. Using the optimal GM (1, 1) gray model, we made a simulation prediction of the surface degradation degree using the overall plane degradation degree as the input, and the correlation between the overall plane degradation degree and the characteristic parameters of the gray distribution map of the CT image was investigated using the gray correlation analysis method.