Through a polyethylene-glycol-assisted hydrothermal method, a series of potassium fluoride (KF)–Yttrium (III) fluoride (YF3) system materials have been synthesized. By controlling the reactant ratios of KF: rare earth ions (RE3+), the hydrothermal temperatures, and the pH values of the prepared solutions, the final products can evolve among the orthorhombic phase of YF3 and/or the tetragonal phase of potassium triyttrium decafluoride (KY3F10) and/or the cubic phase of potassium yttrium tetrafluoride (KYF4). The final products are characterized by the x-ray diffraction (XRD) patterns, the field-emission scanning electron microscopy (FE-SEM) images, the energy-dispersive spectroscopy (EDS) patterns, the photoluminescence (PL) spectra, and the luminescent dynamic decay curves. The XRD patterns of the samples suggest the phase evolution of the final products. The FE-SEM images and the EDS patterns prove that. Europium ion (Eu3+) acting as a probe, its PL spectra and the luminescent decay curves all put together prove the phase evolution of the final products. The research can be extended to study the other KF–REF3 system materials.