Aiming at the problems of high cost and high environmental requirements of traditional slope displacement monitoring methods, a low-cost and high-precision slope displacement visual measurement device and its method are proposed. First, a high-precision camera and a special marker are deployed in the monitoring scene, and the improved YOLOv8-Pose algorithm is utilized to realize the initial recognition of the key points of the marker. Subsequently, a sub-pixel extraction technique is used to process the key point of the marker to obtain its precise coordinates at the sub-pixel level. Next, the pixel displacement change of the marker is calculated by comparing the coordinate offsets of the keypoints at different moments. Finally, the actual displacement change is calculated by the scale conversion method in combination with the known geometric dimensions of the markers, so as to realize the accurate monitoring of slope displacement. In order to verify the practical application effect of the method, this paper selects the slope of a highway in Guizhou Province for on-site monitoring experiments. The experimental results show that the visual measurement method has good accuracy performance in slope displacement monitoring. Compared with the monitoring results of total station, the accuracy of horizontal displacement reaches 90.43%, and the accuracy of vertical displacement is 91.58%, which are more than 90%, fully verifying the feasibility and effectiveness of the method in practical engineering applications.