Abstract: In the grouting reinforcement of water-rich sand layers, the pore structure and spatial distribution of the cemented bodies directly influence reinforcement effectiveness and grout diffusion paths, serving as critical criteria for evaluating grouting quality and optimizing operational parameters. However, due to variations in sand grain size distribution and non-uniform grout diffusion, existing studies remain insufficient in characterizing multi-scale pore structures, particularly lacking systematic validation of the evolution and interrelation of micro–macro pore structures under different sand grain sizes. This study aims to reveal the evolution patterns and spatial distribution characteristics of pore structures in cemented bodies formed in water-rich sand layers with different grain sizes, providing references for understanding grouting mechanisms and selecting engineering parameters. Under consistent grouting materials and injection pressure, simulated grouting tests were conducted on fine, medium, and coarse water-rich sand layers to prepare typical cemented body samples. Micro-scale pore structures were quantitatively analyzed using mercury intrusion porosimetry, while macro-scale pore characteristics were extracted and binarized using MATLAB digital image analysis to calculate porosity. By integrating micro-scale and macro-scale data, a multi-scale pore structure analysis framework was established to systematically compare differences and spatial patterns in the pore structures of cemented bodies across different grain sizes. At the micro-scale, fine cemented bodies are dominated by slit-like and irregular medium-to-large pores, exhibiting high connectivity and the highest porosity; medium cemented bodies show a composite structure of slit-like and spherical pores, with a wide pore size distribution and the most complex structure; coarse cemented bodies mainly develop long columnar or small-to-medium spherical pores, with the lowest porosity, indicating that coarse-grained skeletons favor densification. At the macro-scale, porosity of all three cemented body types increases parabolically with distance from the grout outlet, reaching the minimum at X = 0 cm, with fitting coefficients R² ranging from 0.70 to 0.92, reflecting the spatial characteristics of saturated grout deposition. Micro-scale porosimetry and macro-scale image analysis results were highly consistent, all showing a porosity ranking of “fine > medium > coarse,” validating the reliability of the image analysis method. The study reveals the multi-scale pore structure characteristics and spatial distribution patterns of cemented bodies in water-rich sand layers under different grain size conditions, clarifying the control of grain size on pore morphology, porosity, and densification. The findings provide a scientific basis for understanding grout diffusion and deposition mechanisms, optimizing grouting materials and parameters, and improving reinforcement quality, offering significant engineering applications.