In order to produce chloride-free K-N compound fertilizers by the reaction of KCl (carnallite) and (NH4)(2)SO4 as raw materials, the phase equilibrium of the mutual quaternary system K+, NH4+//Cl , SO42 -H2O and its two ternary subsystems KCl-NH4Cl-H2O, K2SO4-(NH4)(2)SO4-H2O were measured by isothermal method at 273.15 K. And the corresponding phase diagrams were plotted and analysed. The equilibrium phase diagram of the quaternary system K+, NH4//Cl , SO42 -H2O shows that there is one saturation point and three solid phase fields of crystallization. Three fields of crystallization represent solid solution salts corresponding to (K-1 t, (NH4)(t))(2)SO4, (K-1 (m), (NH4)(m))Cl and ((NH4)(n), K-1 (n))Cl respectively. The phase analysis shows that field of (K-1 (t),(NH4)(t))(2)SO4 is larger than other crystalline fields, and there is no crystalline field of pure potassium sulfate, which indicates that the K1-t, (NH4)(t))(2)SO4 is easy to crystallize out from solution. By analysing and calculating the phase diagrams of K+, NH4+//Cl , SO42 -H2O at 273.15 K and 323.15 K, chloride-free K-N compound fertilizers (K-1 (t),(NH4)(t))(2)SO4 can be obtained using KCl and (NH4)(2)SO4 as raw materials at 273.15 K, and ((NH4)(t), K-1 (t))Cl can crystallize out at 323.15 K. The new technology shows the advantages of effective utilization of carnallite resources. The research on this quaternary phase diagram is the foundation of the chloride-free potash production by the reaction of carnallite and ammonium sulfate. (C) 2019 Elsevier Ltd.