The driving force caused by density difference of slurry and drilling fluid is a function of circumference angle and radius in horizontal well displacing. The closer to annulus bottom, the larger is the driving force, which leads to the displacing interface more easily to extend downward annulus. Based on the mechanics analysis of annular fluid displacing infinitesimal in horizontal well, this paper sets up the calculation model of drilling fluid layer dynamic shearing stress in displacing process. According to the equilibrium condition of displacing interface stability, the boundary position of drilling fluid retention layer at casing and sidewall with different circumference angles for horizontal well is acquired, which will enhance the pertinence of displacing techniques in horizontal well.Calculation results show that with the same circumference angle, the thickness of drilling fluid retention layer at sidewall is far larger than at casing. For this reason, the cement bond quality of the second cementing interface should be used as one of criterions for evaluating horizontal well cementing quality. And the retention at sidewall and casing gradually increases from the lower annulus to the upper part. The retention layer at upper sidewall is thickest. So the oil/gas/water channeling-path is easy to form and special attention should be given to cleaning the upper sidewall drilling fluid . Low permeable horizontal wells generally take subsection fracturing method for exploitation , requirements of cementing quality is higher. The guarantee of higher displacing efficiency is the premise of obtaining high quality of cement quality , in the horizontal well because of dirlling fluid retention and free water precipitation , the oil/gas/water channeling-path is easy to form at the upper wellbore , seriously restricted the implementation[1-2] of the horizontal well stimulation measures. At present technology about improving the displacing efficiency develope rapidly[3-6],but because of drilling fluid retention laws and the retention position is uncleared in horizontal well,which lead to these measures are not well targeted. Analysed by displacing interface dynamic extension process in the process of slurry displace drilling fluid , this article establishes dynamic shearing stress calculation model of horizontal well annular fluid displacing infinitesimal , according to the condition of displacing interface stability, the drilling fluid retention law and retention position in horizontal well is acquired.