Transparent oxyfluoride glass-ceramics containing Er3+, Yb3+:Ca1−xLaxF2+x nanocrystals, which may have potential applications in the fields of solid-state laser and luminescence, were prepared. Crystallization of Ca1−xLaxF2+x and behavior of Yb3+ and Er3+ during the heat treatment was investigated. Results showed that alumina content had a significant effect on crystallization of Ca1−xLaxF2+x in the SiO2–Al2O3–CaF2–LaF3 system. Due to the size of phase-separated areas, the size of the crystals during the heat treatment did not change significantly. After crystallization of Ca1−xLaxF2+x in the glass, the majority of Er3+ ions incorporated into the Ca1−xLaxF2+x crystals during the heat-treatment process. Time-resolved luminescence of Er3+ ions in the samples around 842 nm showed that the solubility of Er3+ ions in Ca1−xLaxF3 crystals is higher than pure CaF2 crystals. The glass undergoes an enormous phase separation, which keeps the Yb3+ ions within the other separated phase. Therefore, only at high temperatures (790°C) or with a long heat-treatment time (72 h), there is a possibility for Yb3+ ions to be incorporated into the fluorine phase.