## 1 Introduction  
In high-altitude and high-latitude regions, the underground rock can be frozen to depths of several hundred meters [1]. The stability of frozen rock masses during excavation and mining poses challenges for engineering in such areas [2; 3; 4]. The natural rock mass contains numerous joints and fissures of various types, which directly influence its strength, deformability, and geometric parameters. Therefore, studying the mechanical properties of frozen fissure sandstone holds significant engineering value. Most research on fissured rock masses has been conducted at room temperature. The characteristics of the rock and the geometric parameters of its fissures have a coupled effect on the strength of a fissured rock mass [5; 6; 7; 8; 9; 10], which tends to decrease with increases in both fissure length and width [11]. Additionally, the number of fissures also influences the strength of the rock mass; a higher number of fissures leads to lower initial crack stress and failure strength compared to rocks with fewer fissures [12; 13]. Moreover, as the inclination increases, the uniaxial compression strength (UCS) curve exhibits a decreasing trend followed by an increasing trend, reaching its minimum value at an inclination angle of 30\({}^{\circ}\)[14; 15]. Furthermore, the fissure type also has an important influence on strength. The strength of a rock mass containing parallel fissures is influenced by the inclination and distribution of the fissures, as well as the length and angle of the rock bridge, which can lead to various modes of crack coalescence [16; 17; 18; 19]. In fact, seven types of cracks have been defined for single-fissure-containing rock masses, while the types of crack coalescence are recognized to be combinations of these seven basic crack types [20; 21; 22; 23; 24]. Lastly, it is worth noting that experimental results may be influenced by factors such as filler type and volume within rock fissures; however, overall similarities exist between crack coalescence patterns observed in filled versus unfilled-rock-mass-containing fissures [25; 26]. There has been extensive research conducted on rock mechanical properties at freezing temperature; however, limited studies have been carried out specifically on fissured rock masses. The mechanical properties of frozen rock are significantly influenced by factors such as freezing temperature, moisture content, and pore structure. The strength of frozen porous rock increases as the freezing temperature decreases [27]. The strength of frozen porous rock decreases initially with increasing moisture content, then increases, and finally decreases again [28]. Additionally, freeze-thaw cycling also impacts the mechanical properties of rock masses, with the strength decreasing as the number of cycles increases [29; 30; 31; 32; 33]. Research on frozen fissured rock masses has recently emerged as a focal point of investigation. The ultrasonic acoustic characteristics, local damage, and tip fractures in fissured rock masses during freeze-thaw cycling have been extensively studied [34; 35]. However, there is still a need to investigate the mechanical properties of fissured rock masses at freezing temperatures. Furthermore, it is imperative to explore the mechanism behind the enhancement of freezing in frozen fissure rock mass. The present research conducted experiments to investigate the mechanical properties of fissured rock masses at freezing temperature, specifically focusing on uniaxial compressive strength, strain, and elastic modulus. The failure process was observed using a high-speed camera and acoustic emissions in samples with fissures inclined at five different angles. The mechanism behind the enhancement of strength due to freezing is thoroughly explained. The objective of this research was to examine the mechanical properties and cracking behavior of frozen fissured sandstone, providing valuable insights for safe rock engineering in frozen underground environments.