Solid-liquid phase equilibrium and physicochemical properties of the KCl-KH2PO4-C2H5OH-H2O system at 313.15 K were investigated by the method of isothermal solution saturation, and the equilibrium solids were analyzed by Schreinemaker's method of wet residues and verified by XRD. Drowning-out precipitation of KH2PO4 was carried out by mixing different ethanolic aqueous solutions, and crystal morphology was observed by polarizing microscopy. The phase diagrams exhibit one eutonic point, two univariant curves, and crystallization regions corresponding to KCl and KH2PO4. At all studied ethanol concentrations, the crystallization region of KH2PO4 is larger than that of KCl. The effect of KCl (ethanol) on crystal growth morphology and KH2PO4 product quality was studied.