Abstract: The physical and mechanical property of mud stone largely depend on the microscopic pore structure and the varying pattern in the soil mass. They can adversely affect its engineering characteristic. This paper aims to study the effect of compaction on the micro-porosity of micro-expansion mud stone for high-speed railway. It prepared six kinds of remodeled mud stone samples with different dry densities. It employs freezer dryer to conduct desiccation on saturated samples, and then conducts analytical investigation on micro-pore shape, pore volume, and pore area under different compaction status through mercury intrusion porosimetry. The results indicate that apart from the cylindrical pores exists in the remodeled mud stone, it also contains a large number of bottle neck pores. It proceeds classification on pores with different apertures referring to the pore partition method proposed by shear. It discovers that the pores inside remodeled mud stone are mostly found between particles and aggregates. The compaction performance on microscopic is the transformation from macro-void to small pores caused by the compression of macro-void. The total area of mud stone pores with different degree of compaction is closely identical. The pore area mainly exists in particle. The larger the compaction effect, the smaller the pore area in aggregate. It conducts reasonable analysis and description for the phenomenon. The actual porosity measured with the mercury intrusion method is smaller than that of theoretical calculating result. Linear regression analysis is used to analyze the relationship between measured porosity and calculated porosity, which ensures the correctness and rationality of the measured mud stone micro pore structure through the mercury intrusion method. The research achievement can provide reference frame for expansion mud stone in territorial high-speed railway construction, possess reference significance for similar engineering construction.