The active ions used in laser crystals are mainly transition metal ions and trivalent rare earth ions. The optical electrons of transition metal ions are 3d electrons located in the outer layer. In crystals, these optical electrons are susceptible to the direct effect of the surrounding crystal field. Therefore, their spectral characteristics vary greatly among crystals with different structure types. The 4f electron of trivalent rare earth ions is shielded by 5S and 5p outer electrons, which weakens the effect of crystal field. However, the perturbation of crystal field makes it possible for the forbidden 4f electron transition to produce narrow band absorption and fluorescence lines. So the spectra of trivalent rare earth ions in different crystals do not change as much as those of transition metal ions.The matrix crystals used in laser crystals are mainly oxides and fluorides. As a matrix crystal, besides its stable physical and chemical properties, it is easy to grow large-sized crystals with good optical uniformity and low cost. However, the adaptability between matrix cations and active ions, such as radius, electronegativity and valence state of matrix cations and active ions, should be considered as close as possible. In addition, the effect of matrix crystal field on the spectra of activated ions should also be considered. For some matrix crystals with special functions, laser with certain characteristics can be produced directly by doping activated ions. For example, in some non-linear crystals, laser generated by activated ions can be directly converted into harmonic output through matrix crystals.