Abstract: In order to study the influence of the magnesium chloride in the polluted soils on the early strength of cemented soils after 28 days, unconfined compression tests, X-ray Diffraction (XRD) phase tests and scanning electron microscope (SEM) tests are employed to study the mechanism of cement-reinforced soils contaminated by different content magnesium chlorides. The strength test results show that the compressive strength of the cemented soils decreases with the increase of the content of magnesium chlorides. The value reaches its peak when the content is 1.5 g/kg, while it is the lower when the content is greater than 1.5 g/kg. Chemical products are analyzed by means of the XRD tests, and the test results show that chemical products become more, such as M-S-H, M-A-H, CaCl₂·6H₂O, Mg₂(OH)₃Cl·4H₂O, C₃A·CaCl₂·12H₂O, after the participation of the magnesium chlorides in chemical reactions. Those chemical products play the role of decomposition and crystallization composite action in the chemical reaction process. At the same time, these products change the pore size distribution of cemented soils. From the SEM photos of cemented soil form, the chemical products in the sample with lower magnesium chloride content link the soil particles together, and decrease the porosity. With the increasing content of magnesium chlorides, the gel around soil particles is broken down, and the porosity increases, which make the strength of cement soils decrease.