In the light of annealing data about Cr,Ca:Y3Al5O12 crystals with various Ca/Cr content ratios, we established a set of solid-state defect reaction formulas describing the formation of the four-coordinated quadra-valence Cr4+ laser center in the crystal. The valence state of the Cr4+ laser center depends mainly on the divalent Ca2+ compensation ion in the crystal, which can introduce an oxygen vacancy into the crystal. The concentration of the Cr4+ laser center has also been estimated and indicated that its absorption cross-section at 1064 nm wavelength is most probably 5.7 ×10-18 cm2.