The Anterior Cruciate Ligament (ACL) is one of the four main ligaments of the knee and crucial to stabilizing the knee. In the surgery of arthroscopic ACL reconstruction after ruptures, the precise intraoperative localization of the entry point of the femoral tunnel is one of the key factors affecting the function of knee. In this paper, a new method is proposed to rapidly achieve the localization of the entry point. A special probe should be registered firstly so that its tip position is measured in real time. Then the point cloud of probe tip’s trajectories is obtained when outlining the cartilage posterior edge of lateral condyle. In order to simplify the way to confirm if the entry point of the femoral tunnel is in the preoperative planning position, a projection transformation is applied finally to reduce the dimensions. Besides, the experimental results show that the method can meet the demands of the surgery for ACL reconstruction.