To investigate the strength characteristics and microstructure changes of coarse-grained carbonaceous mudstone (CGCM) soil,this paper analyzes the microstructure of CGCM soil under different loads using mercury intrusion porosimetry (MIP) and scanning electronic microscope (SEM) characterization. The research results show that the microstructure changes of CGCM soil under different loads mainly manifest in the change of particle arrangement. With the increase of confining pressure,the particle arrangement state changes from loose to dense,and the pore volume shows a trend of first decreasing and then increasing,with most of the pores being open and semi-closed. For the CFCM soil with the same gradation,with the increase of confining pressure,the pore roundness first increases and then decreases,while the fractal dimension shows a opposite trend,along with the decreasing pore shape area. The abundance of the four samples under different confining pressures is between 0.46 and 0.51,and the pore shape of the samples are oval. The research results of this paper can provide reference for the strength control of CFCM soil subgrade.