The invention relates to a method for preparing an iodized organic substance or a plurality of iodized organic substances (substance(s)) with the molecular weight less than 2000. (A) At least one substance which can form free radical is used; the substance is chosen from peroxide, diazo compounds, dialkyl diphenyl alkane, the substance derived from tetraphenyl ethane, boron hydride and initiator-transfer agent-terminator which contains at least one disulfide tetra alkyl thiuram radical; (B) ethylenic linkage type unsaturated material is used, the free radical can be added on ylen double linkage by the material; (C) iodide excess is used. The method comprises the following steps that: the at least part (A), the at least part (B) and the at least part (C) are added in a reactor and reacted,meanwhile, (A) possible remainder, (B) possible remainder and (C) possible remainder are led into the reactor till the substance in the reactor becomes a compound which contains one substance or a plurality of substances. The iodized organic substance prepared with the method is very suitable.