Abstract: Clay content is a key factor affecting the engineering properties of red clay. However, the effects of clay content on the strength, deformation, and permeability characteristics of red clay remain insufficiently understood. To investigate the influence of varying clay contents on the mechanical properties of red clay and the evolution of its internal microstructure, Guilin red clay was selected as the research object. Clay particles were extracted from red clay based on the principle of sedimentation in still water, and red clay specimens with different clay contents were prepared. Macroscopic tests, including triaxial shear tests, compression deformation tests, shrinkage deformation tests, and variable-head permeability tests, were conducted to evaluate the effects of clay content on the strength, deformation, and permeability characteristics of red clay. Meanwhile, scanning electron microscopy (SEM), nitrogen adsorption tests, and mercury intrusion porosimetry (MIP) were employed to investigate the influence of clay content on the microstructural characteristics of red clay. The results indicate that with increasing clay content, the cohesion of red clay significantly increases. The enhancement of cohesion effectively compensates for the adverse effect caused by the reduction in internal friction angle, resulting in an improvement in shear strength. Meanwhile, the secant modulus and compression modulus increase, whereas the compression coefficient, linear shrinkage ratio, volumetric shrinkage ratio, and permeability coefficient decrease significantly. Clay particles reconstruct the pore structure through a filling-cementation coupling mechanism, leading to a denser soil structure and improving the strength, deformation resistance, and seepage resistance of red clay. This study not only contributes to improving the theoretical framework of constitutive models for red clay but also provides a scientific basis for engineering design, disaster prevention and mitigation, and foundation treatment in red clay regions.