A series of K0 （coefficient of earth pressure at rest） consolidated undrained triaxial shear tests, scanning electron microscope, and mercury intrusion tests were conducted on subgrade soft soil in Wuhan region. It aims to study the microstructural evolution and damage mechanism during the loading process from macro and micro perspectives. Microstructure parameters were analyzed based on fractal theory and image processing technology. The results show that soft soil pores have multifractal characteristics, according to which the pore size boundary points of the micro, small, medium and large pores are determined to be 0.03, 0.50, and 7.00 μm, respectively. Pores with diameters from 0.50 to 7.00 μm exhibit stronger self-similarity overall. Therefore, 0.50-7.00 μm is defined as a characteristic interval, and the surface fractal dimension decreases with increasing strain in this interval. The microscopic damage of soft soil is characterized by aggregate disintegration, together with the evolution of pore size distribution and pore morphology. With increasing strain, the pore area ratio and shape factor manifest nonlinear increase and decrease, respectively. The microstructure evolution process can be divided into three phases: structural fine-tuning, damage development, and structural failure. The microstructure factor proposed in this study can effectively reveal changes in macro mechanical properties of soft soil.