2-(dinitroethylene)-4,5-imidazolidinedione, which is the intermediate in preparing FOX-7, was synthesized by acetamidine hydrochloride and diethyl oxalate. Ring cleavage reaction could be carried out more quickly, when methanol, water, and formic acid were used as ring cleavage reagents; the disadvantages from using ammonia as ring cleavage reagent could be overcome with yields of up to 65.5%. When water is used as ring cleavage reagent, the reaction rate will be high and the reaction conditions will be milder. So this method is more suitable for further up-scaling research.