Abstract: To understand the features of the cement displacement interface in the horizontal section of the eccentric annulus and to improve horizontal section eccentric annulus cement displacement efficiency, a three-dimensional dynamic displacement mathematical model in horizontal section was established considering flow regime coupling and mass diffusion. The cement displacement interface morphology during cement displacement of the spacers was numerically simulated considering the coupling of eccentricity and density difference. The results showed that the eccentric effect increases with the eccentricity. Further, the fingering trend of high-side displacement interface was enhanced. When the positive density difference increased, the buoyancy effect and consequently the fingering trend of the low-side displacement interface were reinforced. Determining a suitable eccentricity and density difference coupling allowed a critical equilibrium state between the eccentric effect and buoyancy effect. Under these conditions, the displacement interface morphology was relatively stable; the interface length appeared to not significantly increase, and the displacement efficiency reached the maximum value. The simulation results could provide theoretical bases for improving the cement displacement efficiency of horizontal section eccentric annulus.