Ultrafast carrier dynamics in Al2O3/SiO2 high reflectors was investigated by UV femtosecond laser. It was identified by laser spectroscopy that, the carrier dynamics contributed from the front few layers of Al2O3 played a dominating role in the initial laser-induced damage of the UV reflector. Time-resolved reflection decrease after the UV excitation was observed by pump-probe experiment, and the peak value of the variation of the reflectivity of the probe light changed from 417 nm to 402 nm in 2.3 ps. To interpret the laser induced carrier dynamics further, a specific theoretical model including multiphoton ionization(MPI), avalanche ionization(AI), and the mid-gap defect state was built to simulate the evolution process of the electron density in the conduction band, it pointed out that during the conduction band free electron relaxation process, the mid-gap defect state locating about one photon below the conduction band was formed because of the interaction between electrons and lattice. The initial electron density effect of the mid-gap defect state has important influence to the damage threshold of the Al2O3/SiO2 high reflectors. This model agrees very well with the experimental results.