Abstract: Due to special formation process and environment, red clay belongs to peculiar soil with different engineering characteristics. We soak the improved soil into different pH of solutions to study the mechanical properties and action mechanism of improved soil under different dosage, soaking time and pH values. By electron microscopy scanning, we explore the microstructure of modified Guilin red clay and its relationship with mechanical properties. The results show that on macroscopic view, the cohesive force of improved red clay reduces with the growth of soaking times under acid environment. From microscopic view, the overlapping degree, overlapping way as well as cementation forms of soil particle determine the physical mechanical property of soil. Under various pH environments, the red clay structure changes, and the degree of cementation between soil particles is improved.