Luminescent materials with an efficient single-(pure) color up-conversion luminescence (UCL) are expected to be applied to three-dimensional (3D) display, lighting, biological imaging, promoting plant growth and improving the photoelectric conversion efficiency of solar cells. In this work, perovskite-type KMnF₃ fluoride nanocrystals (NCs) are grown in situ in a fluorosilicate glass co-doped with rare earth (RE) ions Yb³⁺/Er³⁺ by a controlled thermal treatment. Compared with precursor glass (PG), the nano-glass composites (also referred to as glass ceramics, or GC in short) thus obtained exhibit a significantly enhanced (by 6 times) red UCL emission. Although a weak green UCL emission can be also observed in the GC, the intensity ratio of the red UCL emission to green UCL emission is as high as 30, implying a good color purity. It is suggested that the dramatic enhancement of UCL emissions in the GCs is due to the doping of RE ions into the KMnF₃ NCs with a much lower phonon energy (330 cm^–1) than that of the silica glass matrix about 1100 cm^–1. However, the doping mechanisms of RE ions into KMnF₃ nano-glass composites are not yet conclusive, mainly because of the charge and ionic radius mismatch between RE ion dopants and cations of KMnF₃. This work combines the high-resolution transmission electron microscopy (HR-TEM) analysis technology and the first principles calculation, to unravel the doping mechanism of RE ions in KMnF₃ nano-glass composites. First, the HR-TEM study provides straightforward evidence that RE ions are preferentially accumulated in KMnF₃ NCs embedded in the glass matrix. Then, through the first-principles calculation considering the charge balance, it is found that the formation energy of RE ions substituting for K⁺ is lower than for Mn²⁺ lattice sites in KMnF₃, which is most likely related to the fact that the ionic radius of the eight-fold coordinated K⁺ is larger than that of the six-fold coordinated Mn²⁺ and thus is more conductive to accommodating the large size RE ions. The electronic densities of states at the top of the valence band and the bottom of the conduction band of KMnF₃ increase after doping the RE ions. It is inferred from the profile of partial density of state that RE ions have a strong bonding tendency with F^– in the crystal. Benefiting from the efficient energy transfer between RE ions and Mn²⁺ in KMnF₃, the green UCL emission is dramatically quenched, and consequently, the GC is endowed with a highly pure red UCL emission. The present study is expected to deepen the understanding of RE ions doping mechanisms in NCs and facilitate the design of highly efficient UCL materials based on nano-glass composites.