Objective To investigate the clinic effect of CT guided percutaneous I125 implantation in treating advanced stage lung mediastinum tumor. Methods 70 cases with advanced stage lung mediastinum tumor in our hospital from February 2013 to June 2016 were selected. Control group was given common treatment and observation group was given CT guided percutaneous I125 implantation. Compare the efficiency, focus control rate and adverse reactions of the patients. Analyze the technology and effect of CT guided percutaneous I125 implantation in treating advanced stage lung mediastinum tumor. Results Efficiency of observation group after 60, 90 d were 27 cases (77.14%), 33 cases (85.71%). 60d after treatment, total efficiency of observation group was better than control group (χ2 =6.119, P=0.013<0.05), 90d after treatment, observation group was better (χ2 =4.786, P=0.029<0.05), the difference was statistically significant; control rate of 3-5cm focus of observation group (90.0%) was better than control group (63.16%), the difference was statistically significant (χ2 =4.852, P=0.0.28<0.05). Particle migration, losing and other complications were not occurred in observation group, 2 cases of myelosuppression occurred in control group. CT guided percutaneous implantation had high accurate rate and better intuition. Conclusion CT guided percutaneous I125 implantation in treating advanced stage lung mediastinum tumor can control focus better, it has better effect and is worthy of wide application.