The invention belongs to the technical field of chemical engineering, and relates to the chemical synthesis technology. The preparation method adopts a cellosolve, which has a good soluble characteristic for the raw materials, as the solvent, and then forms alkaline environment through hydrazine hydrate and ammonium thiocyanate so as to make nucleophilic addition reactions and rearrangement reactions carry out in a same solvent system. The preparation method takes ammonium thiocyanate and hydrazine hydrate as the raw materials; then the ammonium thiocyanate and hydrazine hydrate carry out nucleophilic addition reactions and rearrangement reactions, and finally the reaction product is subjected to a post treatment so as to obtain the thiosemicarbazide; wherein the nucleophilic addition reactions and the rearrangement reactions are carried out in the same solvent at a temperature of 120 DEG C to 135 DEG C. The preparation method of thiosemicarbazide has the advantages of simple operation, easy control, short reaction period, little solvent consumption, and environment-friendliness.