To investigate the enhancement mechanism of atmospheric-pressure oxygen pulsed discharge in a parallel-plate dielectric barrier discharge (DBD) with microstructures fabricated on the dielectric surface of the highvoltage electrode, this paper systematically analyzes the electron transport processes, the formation and evolution of electric fields, and the spatial distribution of particles using a two-dimensional fluid model. The introduction of microstructures induces significant electric field distortion, generating a strong transverse electric field that locally confines and focuses electrons beneath the micro-structured region, leading to the formation of a stable corona-mode discharge. Simultaneously, the reduced local discharge gap near the microstructure enhances the longitudinal electric field, resulting in a temporal asynchrony between the corona discharge under the microstructure and the parallel-plate discharge in the adjacent flat regions. As the geometric dimensions of the microstructures increase, a secondary discharge is triggered, further modulating the overall discharge behavior. Under conditions where the corona discharge is suppressed due to higher protrusions, the secondary discharge effectively compensates by increasing both the high-energy electron fraction and the spatially averaged density of reactive oxygen atoms. Simulation results reveal that the corona discharge and the secondary discharge significantly elevate electron density, electron temperature, and the proportion of highenergy electrons, thereby intensifying the discharge activity. These findings provide deep insight into the micro-mechanisms of microstructure-induced discharge enhancement and offer valuable guidance for the design of highly efficient plasma devices with tailored geometric features.