The key to ensure the cementing quality of small borehole is how to replace drilling fluid and wellbore mud cake in annuli with cementing fluid completely. The complexity of slim-hole conditions, such as irregular borehole and eccentric casing, property of displacement fluid and displaced fluid, cementing parameters are the main factors affecting displacement efficiency, so it is necessary to carry out the analysis of the displacement efficiency of small borehole cementing. In this paper, based on the theory and method of computational fluid dynamics, a numerical simulation model is established to analyze the characteristics of the two-phase interface between displacement fluid and displaced fluid during the cementing process, and to reveal the displacement mechanism of the small hole cementing; Secondly, based on the numerical model of wellbore displacement, the changing process of displacement interface of power-law fluid in wellbore displacement is simulated, and the variation law of displacement interface length and displacement efficiency with displacement time is obtained. According to the numerical simulation results, the effect law of liquidity index and consistency coefficient, density difference between displacement fluid and displaced fluid, casing eccentricity and displacement on displacement interface length and displacement efficiency are analyzed. The research results guided the successful cementing construction of the first sidetracking horizontal well in Changqing Sulige Gas Field.