Abstract: For Zhanjiang clay, the free iron oxide colloids, amorphous iron oxide colloids and using the selective chemical solution method by immersion, improved permeameters and flexible wall permeameters, chelated iron colloids in it are removed. In this analysis, the physical and mechanical properties, mineral composition and structural changes of Zhanjiang clay are compared before and after removing the iron oxide colloids. The interface activity and cementation characteristics of iron oxide colloids and their interaction with clay minerals are discussed. The results show that Zhanjiang clay has the properties of high sensitivity and strong structure owing to the electrical attraction of clay minerals——iron oxide colloids and their cemented connection in pores. The iron oxide colloids have great effects on the properties of Zhanjiang clay, including water stability, consistency, adhesion, permeability, compressibility, sensitivity and structural strength. The interaction mechanism between the iron oxide colloids and the clay minerals leads to change of the basic properties of clay, which is not achieved by changing or destructing the mineral lattice structure, but by controlling the microstructural morphology by changing the coupling states of soil particles and inter-particle forces. As for the iron oxide colloids, both their effects on the soil properties and the mode of occurrence are closely related. Besides the free iron oxide, the amorphous iron oxide is the other important component of microstructural cemented connection of Zhanjiang clay.