Using the WRF-CMAQ (Weather Research and Forecasting-Community Multiscale Air Quality) model and its adjoint (referred to as ADJOINT) model, a simulation is conducted for a 10-d continuous ozone (O3) pollution event in Hainan from 21 to 30 September 2019. The simulation aims to perform source apportionment for the O3 pollution event, quantifying the contributions of emission sources of different areas and species to the event. The results are outlined below. (1) During this pollution event, the mean value of mass concentration of MDA8-O3 (maximum daily 8-h average-ozone) is 167 μg[DK]·m-3, with the peak mass concentration of MDA8-O3 reaching 186.1 μg[DK]·m-3. (2) The WRF-CMAQ/ADJOINT model shows a good ability to simulate the variation of O3 mass concentration during this pollution event in Hainan. The ADJOINT model indicates that the long-distance regional transport is the main source of O3 pollution during this event, with an average contribution of 85% from external emissions and 15% from local emissions. The contribution of external emissions to Hainan is concentrated in the Pearl River Delta region. (3) Through analyzing the emission sources of VOCs (volatile organic compounds), it can be concluded that isoprene has the highest contribution among the sources, with an average contribution ratio of 51%. During the O3 pollution event, the pollution in Hainan is mainly controlled by NOx, and only in Haikou, the pollution is controlled both by VOCs and NOx. Given that the long-distance regional transport is the main source of the O3 pollution event, the future joint control in Hainan and the Pearl River Delta region is crucial for improving the air quality in Hainan.