Abstract: In borehole monitoring for deep displacement, the abrupt characteristics of inclinometer curves are the key basis for identifying the sliding surface. Previous studies have summarized several types of inclinometer curve patterns that exhibit significant sliding surface signals, including "B," "D," and "r" types. For "D" type inclinometer curves, the convex point of the curve is typically used as the position of the sliding surface. However, this method is susceptible to the influence of observation points spacing and coordinate observation scales, which can result in an unclear definition of the sliding surface position and uncertain numerical values. To overcome these drawbacks and improve the accuracy of sliding surface identification in inclinometer curves, a generalized model of landslides composed of “sliding body”, “sliding interval,” and “immovable body” was developed based on the variation characteristics of the "D" type inclinometer curve. A mechanical strut model subjected to external loads was established based on the different flexural rigidity of the three members, and the deformation characteristics of soil at different depths during landslide movement were analyzed in depth. The study found that the sliding surface of "D" curve does not entirely penetrate, leading to continuous soil deformation along the depth direction without abrupt change. The demarcation point of the positive and negative bending moments in the mechanical model is a location where the horizontal displacement of deformation curve is the largest, which can reflect the real deformation characteristics of the landslide and the position of the sliding surface. By converted the accumulated soil displacement into relative displacement, the "D" type inclinometer curve is transformed into "S" type relative displacement-depth curve, and the inflection point of the "S" type curve is close to the position of the sliding surface. By extracting the average relative displacement of soil at different depths during the monitoring period and calculating the depth value of the inflection point in the "S" type segment by using the cubic spline interpolation method, the position of the sliding surface can be determined more accurately, which can greatly improve the reliability and accuracy of deep displacement monitoring and has significant practical value.