Abstract: For highways, how to address expansive soils is one of the important problems, and it is also one of the most prominent geologic disasters in the engineering construction of highways because they shrink and swell according to their moisture content. By means of polarizing microscope, the microstructure of series of weak expansive soil samples is studied, and the classification according to the microstructure features of expansive soils in Yunnan Province is presented. The effects of microstructural feature on expansive soils are evaluated. Moreover, the internal factors for expansion and contraction and the relationship between microstructures and engineering properties of expansive soils were analyzed. The microscopic images show that the arrangement of soil particles can be divided into turbulent flow structures，laminar flow structure，granular structure and cement structure. The constituents of soil particles include matrix，aggregates and transitional links to objects. Due to the loose particles and the large porosity, the water can penetrate and drain out easily to create the necessary conditions for moisture migration of the aggregates, which explains the reason of swell-shrink of the expansive soils based on the microstructure. The microscopic images of the modified expansive soils display that the lime and fly ash have the flocculation effect on the cohesive particles to change soil particles in the microstructure, and to reduce the cohesive particles，and the increasing calcium ions caused by lime and fly ash make the connection more strong, so that they reduce the potential of expansion and contraction. The engineering properties of the modified expansive soils are also improved.