Abstract: Using three-dimensional air quality model WRF/CAMX and chemical process analysis tools,the sensitivity of ozone generation at representative sites in the central and Western Pearl River Delta was quantitatively studied by ratio of formation rate of hydrogen peroxide to nitric acid P_H2O2/P_HNO3 and HOx chain length （HOx_CL）.Before and after the National Day of 2018,the ozone pollution events happened in the central and Western Pearl River Delta.Its P_H2O2/P_HNO3 and HOx_CL had a good inverse correlation.Whether in cities or suburbs,with the rise of height,HOx_CL would decrease.The ozone formation regime in the urban near ground areas was VOCs sensitive,and it was mainly NO_X sensitive above 100m.In the near ground area,the ozone formation regime in Guangzhou and Jiangmen was VOCs-sensitive,and it was NO_X-sensitive in most of the other areas.In this pollution event,most of the ozone in the central and Western Pearl River Delta was mainly generated in the NO_X-sensitive situation.In order to reduce the regional ozone pollution in the Pearl River Delta in autumn,it was suggested that all regions of the Pearl River Delta should reduce NO_X emission.