The invention relates to a potassium nitrate crystallization system and process through a double-decomposition method. The potassium nitrate crystallization system comprises a dissolving tank, a potassium nitrate vacuum crystallization device, a thickener, a first centrifuge, a mixing tank, a concentration device, a potassium chloride crystallization device and a second centrifuge. The potassium nitrate crystallization process comprises the following steps: adding circulating mother liquid, water and potassium chloride into the dissolving tank, sequentially feeding material liquid into the potassium nitrate vacuum crystallization device, the thickener and the first centrifuge, separating potassium nitrate crystals from the first centrifuge, drying and cooling, thereby obtaining the finished product potassium nitrate; adding potassium nitrate crystal mother liquid into the mixing tank, adding ammonium nitrate, uniformly mixing, and sequentially feeding the mixture into the concentration device, the ammonium chloride crystallization device and the second centrifuge; separating ammonium chloride crystals from the second centrifuge, drying and cooling, thereby obtaining the finished product ammonium chloride. A set of novel potassium nitrate crystallization system and process through the double-decomposition method are disclosed by the invention, the stability of potassium nitrate crystallization is improved, and the production quality and the yield are effectively ensured.