Ammonium- containing salts, e.g. NH4- doped sylvite and NH4- doped carnallite, usually form during the evaporation of brines with high NH+4 content. Solid solutions or double salts have been proposed for interpreting the co- precipitation phenomenon of ammonium and potassium in carnallite. However, the mechanism of NH+4 into carnallite deposit has never been well understood. Revisiting the problem critically is of importance for deeply understanding the geochemistry behavior of NH+4 in evaporites. In this study, we revisited the equilibrium of carnallite (possibly KMgCl3·6H2O, NH4MgCl3·6H2O, K2NH4Mg3Cl9·18H2O, KNH4Mg2Cl6·12H2O and carnallite- type solid solutions) with aqueous phase in the quaternary KCl + NH4Cl + MgCl2+ H2O system from experimental and thermodynamic modeling. Besides, the evaporation experiments of the Nanyishan oil- field brine were performed at room temperature to make a comparison with the phase equilibrium results. Our results support the formation of two types of carnallite solid solutions (K- enriched and NH4- enriched). There is no pure KCl, NH4Cl, KMgCl3·6H2O and NH4MgCl3·6H2O in the quaternary system at 298.15 K. Our brine evaporation results also agree with the experimental and thermodynamic modeling results of phase equilibrium, indicating the formation of a NH4- enriched carnallite solid solution instead of new carnallite- type double salt.