Chis paper reports the calculation results of (CrO6)9- transition metal cluster on D3d symotetry site by means of spin unrestricted MS-Xa method. It gives out the single electronic eigenvalues and wave function, discusses the electronic structure of Y3Ga6O12:Cr2+ laser crystal and calculates the 10Dq crystal field parameter and the energy splitting due to low symmetry distortion of the lattice site by Slater transition state concept. The calculation results are in good agreement with the experimental results.