During lung cancer radiotherapy, tumor motion caused by respiration and heartbeat significantly affects radiotherapy efficacy. The real-time and high-precision tracking of the lung tumor during the radiotherapy process remains a big challenge, as the traditional tracking methods have disadvantages such as slow tracking speed, radiation issue, or tracking, and radiotherapy cannot be carried out at the same time. In this study, a magnetic tracking technique based on three permanent magnets was used to localize the tumor in real-time. Thus, precise lung cancer radiotherapy is achieved by adjusting the acceleration position through the magnetic tracking results. Technical verification shows that the tracked centroid accuracies of the simulated tumor based on both one- and three-dipole models were 1.38 ± 0.83 mm, where the tracked tumor diameter accuracy based on the three-dipole model was 1.51 ± 1.30 mm. The phantom experiment indicates that the centroid tracking accuracies of the simulated tumor based on both one- and three-dipole models were 0.36 ± 0.16 and 2.54 ± 1.02 mm, respectively, where the tumor diameter accuracy based on the three-dipole model was 2.15 ± 0.78 mm. Hence, this permanent-magnet-based tumor tracking technique can effectively improve the performance of robotic radiotherapy. Besides, the radiotherapy object can be further expanded from lung cancer to other cancers such as prostate cancer.