Disclosed is a method and device for simply and safely preparing, using a direct synthesis process at room temperature, a radioactive methyl iodine (CH3131I) tracer for use in evaluating the ability of impregnated activated carbon to adsorb radioactive organic iodine according to ASTM D 3803 (Standard Test Method for Nuclear-Grade Activated Carbon), in which the tracer can be directly synthesized by mixing radioactive sodium iodide (Na131I) with methyl iodine (CH3I) at room temperature under reduced pressure, thus shortening excessive synthesis time and decreasing the probability of radiation exposure due to leakage of volatile material during the distillation.