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A nitrogen purification pilot plant has been developed to meet the requirements of high purity nitrogen used in the Jiangmen Underground Neutrino Observatory (JUNO). The radon content in nitrogen dropped from (20.5 ± 0.9) mBq/Nm3 (statistical error) to (25.2 ± 4.5) μBq/Nm3 (stat.) after purification. The Rn reduction factor of the nitrogen purification pilot plant was estimated to be 813 under the condition of 20 mBq/Nm3 of radon activity. Another experimental result showed that the radon activity dropped from (61.6 ± 7.5) μBq/Nm3 (stat.) to (19.2 ± 2.1) μBq/Nm3 (stat.) after nitrogen flew out from the nitrogen purification pilot plant. It shows that the radon activity of nitrogen after purified by the nitrogen purification pilot plant meets the cleanliness requirement of JUNO and the Rn reduction factor of the nitrogen purification pilot plant would decline when the radon activity of raw nitrogen declines to dozens of μBq/Nm3.