The KCl-NH4Cl-H2O system is common in the potash industry, but it is difficult to obtain pure KCl in this system because of the mixed crystals of KCl and NH4Cl. Therefore, phase diagrams of the KCl-NH4Cl-NH3-H2O system at (273.15 and 298.15) K were determined in this paper. The results showed that the phase diagrams of the KCl-NH4Cl-5 % NH3-H2O system at (273.15 and 298.15) K, KCl-NH4Cl-10 % NH3-H2O system at 273.15 K, and KCl-NH4Cl-15 % NH3-H2O system at 298.15 K were consist of four crystallization fields, that are, NH4Cl, NH4Cl + (K, NH4)Cl, (K, NH4)Cl, and KCl, respectively. However, for the KCl-NH4Cl-15 % NH3-H2O system at 273.15 K and the KCl-NH4Cl-23 % NH3-H2O system at 298.15 K, there are only three crystallization fields corresponding to NH4Cl, NH4Cl + KCl, and KCl. It can be concluded that, with increasing NH3 contents, the crystallization area of the mixed crystal disappears gradually. To clarify the mechanism of the phase field changes, a Fourier transform infrared spectroscopy (FTIR) method was used to monitor the bond changes in the dissolution process. The results of the FTIR method showed that new hydrogen bonds between NH4+ and NH3 in the solution were formed with adding NH3 in NH4Cl solution. Therefore, it was speculated that the solubility of NH4Cl would increase with the adding of NH3. While, when adding NH3 in KCl solution, H2O and NH3 would associate, which led to the decrease of KCl solubility.