At present, multi-stage flash ( MSF ) crystallization technology is used in the four-step closed cycle production of potassium nitrate process. The production process of potassium nitrate and MSF crystallization process by four-step cycle of ammonium nitrate and potassium chloride as raw materials were discussed. Production practice proves that using the four-step closed cycle method, the annual output of potassium nitrate is 50 000 tons; using MSF crystallization technology, the product quality can reach the national first-class requirements, the whole system basically has not waste discharge, and the technology has the advantages of energy saving, good product quality and so on.