[Purposes] The study is to further study the influencing factors of shear strength of expansive soil and explore the mechanism of the most essential swelling-induced characteristics of expansive soil on shear strength. [Methods] On the basis of summarizing the main research results on the shear strength characteristics of expansive soil at home and abroad, this article categorized the influencing factors of the shear strength of expansive soil into three special properties: swelling shrinkage, cracking, and overconsolidation. Based on the theory of unsaturated soil shear strength, a scientific hypothesis was made regarding the changes in effective shear strength indicators of expansive soil before and after swelling, and experimental verification was conducted using an improved direct shear test device and a laser particle size analyzer. [Findings] The variation of shear strength of expansive soil is closely related to its three special properties; In the saturated direct shear test, the effective cohesion and effective internal friction angle both decrease hyperbolically with the decrease of dry density; After swelling, the content of particles with a particle size below 0.1 mm increases significantly, and the greater the dry density, the greater the cumulative content change. [Conclusions] The micro mechanism of strength attenuation in expansive soil is due to the fragmentation and separation of solid particles in expansive soil under the action of lattice swelling, resulting in changes in particle gradation. This is an important factor causing differences in strength indicators.