A tumor radiation probe of iodine-123 marker thymidine (FLT) analogue [123I]-IaraU is disclosed. Commercial available uridine is used as the raw material for the synthesis of the precursor. A radioactive iodine-123 is marked on an alkaline group of uridine to obtain [123I]-IaraU, which is distinguishable from [18F]-FLT marking 18F on a glycosyl group to obtain a novel tumor radiation probe. The marking procedures include mixing the marker precursor with Na [123I] solution, acetic acid and hydrogen peroxide solution, and the solution of chloroform and sodium hydroxide. The sonication time increases from 1 minute to 10 minutes, so that [123I]-IaraU has radiologically chemical purity of higher than 98% and radiological specific activity of not less than 0.196 GBq/umole, and the yield can increase from 8% to 40%. Its radioactive specific activity, yield and purity reach to the degree for the use in biological experiments, while reducing production cost.