Rare earth doped fluorite halides play important roles in the fields of economy, national defense, and so on. The trivalent rare earth ions easily cluster in fluorite halides due to the special crystal structure. However, the systematic investigation into the aggregation of rare earth ions is rarely reported. In this work, the cluster structure characteristics, evolution patterns and their connections, as well as the influencing factors were systematically studied. It is found that the stabilities of 1|0|0|11 (C4v) and 1|0|0|12 (C3v) rare earth monomers vary with the ionic radius, and the trends are different in different matrix crystals, which are consistent with the analysis of lattice distortion. The coupling of lattice distortion and coulomb interactions determines the site occupations of charge compensation interstitial halide ions, i.e. the relative stabilities of C4v and C3v center. Additionally, the covalent effect makes the monomer structures of rare earth doped PbF2 and SrCl2 are different from that of CaF2, SrF2 and BaF2. The study also reveals the relationship of energy difference slopes of monomers with crystal ionicity and cell size. The structures of high-order rare earth clusters were also studied and its evolutions are intimately related with that of monomer centers. The criterion of relative stabilities in high-order clusters is proposed, which provides a guideline for the exploration, screening and development of new rare earth doped fluorite halides, and its application scope is highly expected to be extended.