In recent years, O3 pollution in Nanjing has gradually intensified. In order to understand the impact of surrounding cities on O3 in Nanjing, we simulated the air quality in the Yangtze River Delta(YRD) using the source-oriented WRF/CMAQ regional air quality model from March to October 2018.By tracking the emissions from 41 cities in the YRD and outside the YRD, the characteristics of the transportation of O3 in Nanjing were analyzed under different winds. The results show that from March to October 2018, Nanjing contributes 56.8% of local O3 on polluted days, and contributed 49.5% on polluted days. When pollution occurs, the contribution of surrounding cities to Nanjing increased from 36.4% to 46.3%, and the contribution from outside the YRD to Nanjing decreased from 6.8% to 4.7%. From the perspective of daily changes, Nanjing O3 accounts for more than 70% of the local contribution during the daytime, and at night, most of the transport contributions come from surrounding cities and areas outside the YRD (more than 95%) .On all O3 pollution days, the contribution in Jiangsu Province accounts for 73.1%, 10.8% in Zhejiang Province, 10.5% in Anhui Province and 0.95% in Shanghai. During the pollution period, the contribution ratio of O3 (O3N) produced by NOx as a precursor in Nanjing to O3 (O3V) produced by VOCs as a precursor was about 7:13; During all polluting periods, the proportion of southeast and northeast winds was 38.0% and 35.9% respectively, when the dominant wind direction is northeast, Nanjing has the highest concentration of non-background O3, and the contribution of upwind to Yangzhou reached 13.6%. When the dominant wind direction is southeast wind, the upwind to Changzhou (7.3%), Zhenjiang (7.0%) and Wuxi (6.5%) contributed more to Nanjing O3. Therefore, Nanjing O3 pollution control should consider the transportation of upwind to cities.