Soil improvement is a common means in special soil subgrade engineering. In this paper, the main improvement methods were reviewed to promote the development of special soil subgrades. Based on their basic principles， the current soil improvement methods can be divided into two categories: physical improvement and chemical improvement. The advantages and disadvantages of various improvement methods and their applicable areas were analyzed, and the regulation mechanism of special soil improvement was discussed and prospected from the perspective of microscopic bound water. To meet economical and environmental needs, both physical and chemical improvements should consider the local conditions, and the improvement method integrating functions of multiple materials should be investigated to make up for the insufficiency of improvement methods using a single material. The internal mechanism affecting soil strength and other characteristics is revealed， and it is found that physical and chemical improvements can significantly affect the bound water in soil, and the change in bound water may be the main factor affecting soil strength. In the construction of future projects, most of the solid waste can be reused to make things of value. This review can provide basic knowledge and practical methods for different types of special soil subgrades, as well as a decision reference and basis for using special soil as subgrade filling.