A method for preparing 
potassium nitrate and 
ammonium chloride employing double 
decomposition reaction comprises the following steps: dissolving 
ammonium nitrate and 
potassium chloride in water according to a defined ratio at 110 DEG C, continuously adding 
potassium chloride and water, heating while stirring to ensure that 
potassium nitrate is in 
supersaturation state, after stopping heating, cooling the solution in a 
vacuum cooling crystallizer to 36-40 DEG C to separate 
potassium nitrate crystal, placing the 
potassium nitrate crystal in a centrifugal 
machine with a filter cloth lining to obtain coarse potassium 
nitrate, then washing the potassium nitrate with cold water, 
drying to obtain the finished potassium nitrate; in addition, adding 
ammonium nitrate in mother solution I and cleaning solution to adjust solution concentration so that 
ammonium chloride can reach 
supersaturation state, using a vacuum concentration device to perform negative pressure 
evaporation, separating and precipitating 
ammonium chloride by centrifuging and obtaining a 
solid ammonium chloride product, wherein, when dissolving 
ammonium nitrate and potassium chloride, the ratio of ammonium 
ion to 
chlorine ion is 1:2 and when using the centrifugal 
machine to obtain the coarse potassium nitrate, the separated mother solution is another mother solution I sharing the same saturation point of potassium nitrate and ammonium chloride. The solution of feed liquid circular reaction overcomes the defects of the prior art that the price of potassium nitrate used in reaction is high, the resource of potassium nitrate is in short supply and the cost of devices used in 
ion-exchange method is high, thus being applicable to the production of potassium nitrate.