Abstract: According to the carnallite cold crystallization process of Qinghai salt lake megaton potash fertilizer project，the purpose of grain growth of potassium chloride crystal at room temperature was mainly achieved by controlling the decomposition condition of low-sodium carnallite and controlling the degree of supersaturation of potassium chloride in the solution so as to reduce the quantity of potassium chloride crystal.During the crystallization process of potassium chloride，the heterogeneous nucleation，such as degree of supersaturation of solution，bottom flow，mother liquor circulation，solution circulation rate，stirring intensity，crystallization temperature，crystal wear and roughness of inner wall of crystallizer etc.were several factors that influenced the product′s particle size.Through the decomposing solution density control，bottom flow reduction and mother liquor circulation increment，ideal particle size of product was obtained in the experiments.Moreover，the quality and yield of products were improved.As the optimum process control conditions to guide the industrial production，the production cost was reduced greatly and the economic benefit of enterprises was increased thereby.