To improve the accuracy of radiotherapy, it is necessary to monitor the displacement of the patient's focus area in real time during radiotherapy. Considering that the surface of the patient's body is blocked by the thermoplastic film used during radiotherapy, the material and shape of the marker were designed to minimize the impact of this thermoplastic film on the positioning accuracy of the marker. First, the markers covered by the thermoplastic film were segmented according to their color characteristics; thereafter, the convex hull algorithm was employed to calculate the outer contour of the markers. Next, the improved least squares ellipse fitting algorithm was adopted to fit the points on the outer contour of the markers and locate the markers. Lastly, based on the location of the markers, features were matched to the markers, and the patient's displacement value was thus calculated. Based on a simulation experiment platform and multiple experiments, this method for measuring patient displacement achieves an accuracy of 0.2 mm, with frame rates of up to 30 Hz. Thus, this approach can meet the requirements of real-time and accurate displacement monitoring in radiotherapy.