Abstract: The drilling fluid retention caused by casing eccentricity in horizontal well restricts the improvement of cementing quality and the advantage of horizontal well. To accurately describe the retention law and range of drilling fluid in horizontal well annulus, It was considered in the paper of the effect of driving force caused by density difference between slurry and drilling fluid in condition of casing eccentricity in horizontal well on extension of displacing interface. Based on the equilibrium condition of displacing interface stability, the boundary location calculation model on displacing interface of drilling fluid at casing and well in eccentric annulus was set up, and the effect of casing eccentricity on displacement in horizontal well was analyzed. The results showed that the displacement efficiency decreased and the thickness discontinuity of annular cement sheath increased as the increase of eccentricity when casing upper eccentricity, while the displacement efficiency increased and the thickness of annular cement sheath tended to be uniform at first and to be uneven as increase of eccentricity when casing lower eccentric. The casing lower eccentricity of 0.1 to 0.2 in horizontal well can bring about the best cementing quality, so the casing upper eccentric should be strictly controlled to prevent whole drilling fluid retention and uneven distribution of cement sheath thickness.