Text：Apatite is a versatile and important mineral group with significant roles in geology, biology, and industry. This study employs density functional theory (DFT) to investigate the electronic structures and optical properties of three centrosymmetric (CS) crystals: Ca 10(PO 4) 6F 2(FA), Ca 10(PO 4) 6Cl 2(ClA), and Ca 10(PO 4) 6Br 2(BrA), and two non-centrosymmetric (NCS) crystals: Ca 10(PO 4) 6(OH) 2(HA) and Ca 10(PO 4) 6O(OA). Despite the strong structural resemblance, the optical properties of OA show discrepancy among other apatite analogs. Specifically, the bandgap of OA is reduced from over 6.0 eV to 4.328 eV, and a significant second-harmonic generation (SHG) response ( d 11 = 0.824 pm/V and d 22 = 1.327 pm/V) compared with NCS analog HA ( d 15 = -0.021 pm/V and d 33 = -0.026 pm/V). The main reason for these disparities is that in other compounds the bonds between Ca and X tend to be ionic, whereas in OA, the bonds formed between Ca and O C exhibit covalent character, and almost all of the charge obtained by O C comes from Ca. This not only reduces the bandgap but also enhances the internal charge transfer capability, leading to an increase in the SHG coefficient.