Background：The influence of the limit ring on the final dose distribution in the design of the lung cancer intensity-modulated radiation therapy (IMRT) plan was studied. Materials and Methods: A total of 20 patients with lung cancer in 2017 were selected. Seven radiation beams were designed for each patient, and the limit ring width (RW) was set at 0.6, 0.8, 1.0, and 1.2 cm, respectively. The distance between the inner diameter of the limit ring and the target area (RD) was set 0.6, 0.8, 1.0, and 1.2 cm, respectively. The other parameters used in the plan were set at the same position. In addition, the conformity index (CI) and homogeneity index (HI) were calculated, and normal tissues were assessed. Results: Under the condition of the same number of radiation beams and the same distance between the limit ring and the target area, the smaller the limit ring, the better the CI of the target area and the less the HI of the target area. When the size of the limit ring was set the same, the closer the limit ring was to the target area, the better the CI of the dose in the target area, and the less the HI of the target area. Conclusions: In central lung cancer, when the target volume is approximately 800 cc, the optimal dose distribution is obtained when the RW is set at 0.8 cm and the RD is set at 0.6 cm.