###### Abstract  
The deterioration mechanism of red-bed soft rock is attributed to microstructural changes induced by water-rock interactions, where the damage to pores and the development of fractures significantly impact its macroscopic mechanical performance. The microstructural deterioration pattern of red-bed rock with different water saturation times was obtained by scanning electron microscopy (SEM), which showed that the microdamage effect of mudstone is an evolutionary process comprising the dissolution and detachment of its internal soluble bond structure. Three-dimensional (3D) images of the water-saturated damage of the rock mass were extracted by computed tomography (CT) scanning and calculated based on the box-counting algorithm to obtain the fractal dimension of the fracture development, leading to the theory that a soft rock fracture has multiple fractal characteristics. With the continuous increase in the water saturation time, the pores inside the rock mass began to reorganize and connect, and the mudstone specimens showed accelerated damage and some self-similarity. Furthermore, chemical element testing was conducted on the solutions used for water saturation, leading to the establishment of a microscopic interfacial evolution model describing water-rock damage, with theoretical equations derived based on diffusion theory. R 2024 14 4458 1000 10  
in structural damage to red-bed soft rock. This destruction of the macroscopic structure is the cumulative manifestation of microstructural damage. The structural change rule of red-bed soft rock can be obtained by wet-dry cycle tests. Furthermore, the structural change process of red-bed soft rock can be observed based on image technology, such as CT scanning, SEM, etc. (Yao et al. [10]; Zhou et al. [11]; Zhou et al. [12]; Zhang et al. [13]). The strength variation of soft rock has been extensively researched, with a particular focus on compressive strength and shear strength. These studies have established a comprehensive mechanical testing system for the investigation of the water-rock interaction, elucidated the underlying strength change rule in soft rock's water-rock interaction and proposed a constitutive model to describe the strength change (Mandal et al. [14]; Huancollo et al. [15]; Alekseev et al. [16]; Jaiswal et al. [17]). Fractal theory was first proposed by the French mathematician Benoit Mandelbrot in 1975 and is mainly used to describe the many objects in nature with an irregular geometry (Mandelbrot [18]). These objects have a disordered geometry and no regular geometric features, and fractal theory is suitable for describing this class of objects. It uses self-similarity theory to describe the irregular geometric objects in nature from local to whole, from ordered to disordered and from finite to infinite. Therefore, fractal theory can effectively describe nonlinear problems, and a large number of researchers have applied fractal theory to describe the morphology of fractures and evaluate the cracking mechanisms (Hewage et al. [19]; Du et al. [20]; Chen et al. [21]). The deterioration of rocks is characterized by the generation and evolution of fractures that are irregular and disordered. Consequently, the irregular evolution of rocks in the deterioration process can be characterized by means of three-dimensional fractal dimensions, thus enabling the complexity of the arrangement of the fractures to be studied (Zhang et al. [22]). The preceding studies reveal that the majority of existing research has focused on qualitative analysis, with a notable absence of studies investigating the nature of the water-rock interaction in soft rocks and the evolution law of water-rock interfaces. However, the generation and development of microscopic fractures in soft rock have been identified as playing a pivotal role in the mechanical properties of these materials. To address the aforementioned issues, this study employs SEM and CT scanning techniques to quantitatively analyze microscopic waterstone damage in soft rocks based on fractal theory.