The optical radiation of sapphire during shock compression process is closely related to its transparency and structural phase transition under high pressure. This study investigates the optical radiation characteristics of sapphire above megabar pressure (namely, 100 GPa) on the loading platform of two-stage light-gas gun using the target structure of direct contact between dense metal and sapphire window. A typical optical radiation signal containing metal-interface radiation is obtained using a multi-channel radiation pyrometer. The radiation through sapphire is determined using the radiative transfer model. Results show that the change in radiation temperature of sapphire in the shock-compression region under the pressure above megabar is found to show a significant turning and the shock-induced structural phase transition of sapphire occurs under the pressure near to 87 GPa.