The invention discloses a continuous synthesis process of guanidine nitrate. The method comprises the following steps: (1), continuously adding materials into a melting kettle according to a proportion, controlling the temperature of the melting kettle, overflowing the materials to enter a primary reaction kettle after the materials are molten; (2), controlling the temperature and the retention time of the primary reaction kettle, continuously ventilating an ammonia gas into the primary reaction kettle, and overflowing the reacted materials to enter a secondary reaction kettle; and (3), controlling the temperature and the retention time of the secondary reaction kettle, continuously ventilating the ammonia gas into the secondary reaction kettle, and cooling, slicing and packaging the reacted materials to obtain a guanidine nitrate finished product. The synthesis process disclosed by the invention is continuously carried out for the whole process and stable to operate, so that the operation strength is greatly lowered, the product yield is high, the quality is good and the yield is high.