6-Chloro-2-diethylaminoethyl-3(2H)-pyridazinone was synthesized by the reaction of 6-chloro-3(2H)-pyridazinone and 2-diethylamionethyl chloride reaction in methylbenzene. Then the structure was characterized by means of 1H NMR, IR, UV. By the method of ab initio HF and density functional theory (DFT) B3LYP, the geometric structures of the reagent intermediate, the product and its isomer were optimized and their total energies were calculated. The properties for the frontier molecular orbitals and the rules for energy distribution were analyzed systematically. It was shown that the energy of the nitrogen alkyl compound is lower than that of the oxy alkyl compound and the former is stable than the latter . This result is in accordance with the fact that 6-chloro-2-diethylaminoethyl-3(2H)-pyridazinone is synthesized by the reaction of 6-chloride-2-diethylaminoethyl-3(2H)-pyridazinone and 2-diethylaminoethyl chloride.