The invention discloses a preparation method of potassium chloride and byproducts thereof, which comprises the steps of respectively preheating bittern and auxiliary raw halogen, then directly entering a -effect evaporation tank for carrying out evaporation, obtaining -effect evaporation completed liquid, separating -effect salt slurry and -effect clean liquid by a -effect salt discharge tank andobtaining the byproduct of -effect salt by carrying out centrifugal separation on the -effect salt slurry; respectively preheating the -effect clean liquid and thick halogen, then entering a -effect evaporation tank for carrying out halogen exchange, evaporation and concentration, obtaining -effect evaporation completed liquid, reducing temperature by a negative pressure temperature cooler, discharging into a temperature-keeping settler for carrying out temperature-keeping settlement after temperature reduction, and separating high temperature slat slurry and supernatant liquid, wherein, the high temperature slat slurry is filtered by a vacuum filter for obtaining the byproduct of high temperature salt, the supernatant liquid is cooled, settled and separated for obtaining carnallite and the thick halogen, and the carnallite is decomposed by adding water and washed for obtaining a potassium chloride product. The preparation method can realize the purposes of high product yield, good quality, environmental protection and high utilization ratio of equipment.