###### Abstract  
Saturation is one of the critical factors causing frost damage to rock masses in alpine regions, and dynamic stress perturbations further complicate the damage process. Therefore, the effects of water content and loadings should be considered in the construction and maintenance of rock structures during winter in cold regions. In this study, the effects of saturation and impact loading on the dynamic mechanical behavior of frozen red sandstone were investigated using a low-temperature split Hopkinson pressure bar system (LT-SHPB). By combining low-field nuclear magnetic resonance (LF-NMR) and scanning electron microscopy (SEM), the dynamic evolution of the microstructure of the frozen sandstone due to changes in saturation was investigated. The results indicated that the increase of saturation reshapes the pore structure of the frozen sandstone and promotes the expansion of pores of different sizes during freezing, while at complete saturation the frozen samples are mainly developed with meso- and macrotopes. The dynamic strength, elastic modulus, and brittleness index of the frozen sandstone under impact loading, which are limited by the critical saturation Src, tend to increase and then decrease with saturation. For the four impact loads, the dynamic strength of the samples increased by 21.2%, 27.1%, 32.5%, and 34.3% when the saturation was increased from 0 to 50%, corresponding to 1.38, 1.43, 1.51, and 1.56 times the dynamic strength of the fully saturated samples, respectively. In contrast, the ultimate deformation capacity of the frozen sandstone showed an opposite trend with saturation. As the impact load increases, the dynamic strength, elastic modulus, and peak strain of the frozen sandstone show a significant strengthening effect due to the increase in strain rate, while its brittleness index gradually decreases, dropping by 11.2% at full saturation. In addition, the energy dissipation capacity of the frozen sample first increases and then decreases with increasing saturation, with the enhancement effect of saturation on energy dissipation smaller than the weakening effect. s 2022 2022 2022 2022 2022 2022 2022 2022 2022 20222 2  
modulus, cohesion, and internal friction angle, are significantly improved [6; 7]. The presence of pore ice is a key factor in improving the properties of rocks at low temperatures [8; 9]. For example, dry rocks show little change in mechanical properties at low temperatures, while the properties of saturated rock (e.g., strength or modulus) show an increasing tendency, with the effect being exacerbated in colder environments [10; 11]. Unlike other factors such as temperature, freezing time, etc., the mechanical response of rocks at low temperatures is closely related to saturation, and their mechanical properties are controlled by the coupling of water weakening and ice solidification. However, when saturation exceeds a critical value, the volume growth of pore ice leads to damage, which is manifested by a rapid increase in local strain and the number of internal microcracks [12]. By performing strain and acoustic emission tests on frozen sandstone with different satu-rations, [PERSON] et al. [7] found that significant strain and acoustic emission signals were present during freezing only when the saturation exceeded 80%. However, current studies on the mechanical behavior of rock at low temperatures have mainly focused on saturated rock, especially under quasi-static loading, and may not fully reflect the mechanical behavior under dynamic loading. The dynamic mechanical characteristics of rocks at low temperatures have attracted more attention with the increase in rock structures in cold regions [13; 14]. The split Hopkinson pressure bar (SHPB) system is the main equipment for dynamic testing [15; 16]. The SHPB test system has been used to investigate the dynamic mechanical properties and deformation behavior of rock under freeze-thaw action [17; 18; 19]. It was found that the dynamic strength and deformation resistance decreased with freeze-thaw cycles [20].