The invention discloses a system and method for synthesizing dicyandiamide by a one-step process. The method comprises the following steps: introducing ground lime nitrogen, water and carbon dioxide gas into a reactor; performing a polymerization reaction at 70-80 DEG C by controlling the flow rate of carbon dioxide under a condition that the pH value is 10-11; filtering the polymerization liquid when the polymerization liquid is hot to remove impurities and residues; cooling and crystallizing the filtrate, and filtering to obtain dicyandiamide crystal and dicyandiamide crystallization mother liquid; and washing and drying the dicyandiamide crystal to obtain a dicyandiamide crystal product, and returning the dicyandiamide crystallization mother liquid to the reactor for recycling. According to the method, dicyandiamide can be directly synthesized by controlling conditions, a cyanamide intermediate is not generated, efficient production of dicyandiamide is realized based on crystal stabilization and high quality, the capacity is improved, and the equipment cost and operation cost can be reduced.