Ethylene thiourea (ETU), with both yield and HPLC purity over 99%, was synthesized with ethylenediamine and CS 2 as raw materials and water as solvent in one pot, and characterized by NMR and FTIR. The effects of CS 2 dropping temperature, molar ratio of ethylenediamine to CS 2 , reflux reaction temperature, reflux heating rate, and NaOH dosage in NaOH reaction section on the yield and purity of ETU were then investigated. The results showed that the optimum synthesis conditions were as follows: CS 2 dropping temperature of 25~30 ℃, n(ethylenediamine)∶n(CS 2 )=1∶1.20, reflux reaction temperature of 65~70 ℃, reflux heating rate of 0.17 ℃/min, and NaOH dosage in NaOH reaction section being 3.0% of the mass of ethylenediamine. 1,3-diethylthiourea (DETU) and tetrahydro-2(1H)-pyrimidine thioketone (PUR) were then synthesized under the optimized process conditions, and the pilot test indicated that the process was stable and reliable with the yield and HPLC purity of products obtained >99%, and the mother liquor could be recycled.