Abstract: Oil-base drilling fluid is widely used for the drilling of deep wells and complex-structure wells owing to its better properties. With its wide application, however, the complicated conditions occur more and more frequently in the hole because of the contact contamination between the oil-base drilling fluid and the cement slurry. In this paper, the effects of the drilling fluid and its additive on the properties of cement slurry were firstly analyzed. Then, the phase and microscopic morphology of cement slurry before and after it was blended with drilling fluid and its additive were investigated by means of modern analysis technologies, e.g. SEM, XRD and FTIR. It is shown that the cement slurry is condensed and thickened for the content of calcium hydroxide is consumed and increased in the different stages of cement hydration by the fi ltrate reducer and the calcium chloride in the oil-base drilling fluid, respectively, and consequently the mobility of the cement slurry gets worse seriously. The primary emulsion, auxiliary emulsion and clay increase the strength and thickness of the film at the interface of oil-base drilling fluid, so the emulsified structures of oil in water/water in oil cannot be formed easily. As a result, the contamination is induced. The systematic study plays a signi fi cant role in understanding the essence of contact contamination between oil-base drilling fluid and cement slurry, providing the technical reference for the prevention of contact contamination between them.