As awareness about the danger of radon in indoor air has increased, various studies have been conducted to reduce the source of radon. This study was performed to investigate the effect of radon mitigation technology in a railway tunnel. Radon barrier paint and radon shield membrane developed to reduce the concentration of radon in soil and construction material were applied in the tunnel. The tunnel was divided into three sections, A, B, and C, and radon barrier paint, a buffer section, and radon shield membrane were applied, respectively. After securing a sealing screen to the floor and division of each section, radon concentrations were measured and compared before and after each product was applied, and statistical significance was confirmed through the Wilcoxon signed rank test. Measurement was performed with the In-Situ Method and Closed Chamber Method. Radon concentration measured by the in-situ method changed in A section to 124.1 Bq/m2/day from 614.1 Bq/m2/day (79.8%, z=-2.521, p<0.05), in B section to 416.2 Bq/m2/day from 467.1 Bq/m2/day (10.9%, z=-0.980, p=0.327), and in C section to 47.3 Bq/m2/day from 645.6 Bq/m2/day (92.7%, z=-2.521, p<0.05). Radon concentration measured by the closed chamber method recorded a decrease in A section to 88.8 Bq/m3 from 364.2 Bq/m3 (75.6%, z=-2.201, p<0.05), in B section to 471.8 Bq/m3 from 583.3 Bq/m3 (19.1%, z=-0.700, p=0.484), and in C section to 115.9 Bq/m3 from 718.8 Bq/m3 (83.9%, z=-2.521, p<0.05). In addition to soil, it is very important to mitigate radon from building materials with a high contribution rate of radon in order to manage radon by source. Due to the spatial characteristics of railway tunnels, soil and wall concrete structures are exposed as they are, so it is considered that radon mitigation actions are required utilizing verified methods with high mitigation efficiency.