Abstract: The strength of cement-soil mixing piles with gypsum is larger than that without gypsum in soft ground treatment. In order to explain this phenomenon, a series of micro-experimental studies on chemical and physical microstructure of cement-mixed soils with and without gypsum in addition to undisturbed soils on a soft ground are conducted by using X-ray diffraction (XRD) and scanning electron microscope (SEM) tests. The results show that there are significant differences in the hydration reaction and physical microstructure of treated soils between with and without gypsum, which directly affects the macro-scale strength. Therefore, in order to optimize the ground treatment in similar conditions, gypsum additive should be properly used to improve the microstructure of cement-treated soils.