Abstract: Based on the O₃ concentration data at 32 air quality monitoring stations in 18 cities and counties in Hainan province from 2015 to 2018, the spatio-temporal variation characteristics of regional O₃ pollution (defined as ≥ 3 cities and counties with O₃ concentration during eight hours exceeding the standard) were analyzed, and the subjective classification of synoptic weather systems causing O₃ pollution was conducted.The results show that 40 days are occurring regional O₃ pollution in Hainan province from 2015 to 2018, with an occurrence probability of 2.73%.In 2015 and 2017, it reaches 13 days, with a probability of 3.56%, the following is 11 days in 2018 (3.01%), and the minimum is only 3 days in 2016 (0.82%).The synoptic systems affecting regional O₃ pollution are classified into four patterns, including cold air westward circulation pattern, cold air eastward circulation pattern, warming high-pressure ridge pattern, and tropical system pattern.The cold air westward circulation pattern is the predominant regional O₃ pollution pattern, occurring for 14 days, accounting for 35% of all days, with heavy pollution.Different synoptic system patterns show different distributed characteristics of regional O₃ pollution.The conditions with downdraft at 500 hPa, northeast wind at a lower level, low relative humidity area extending from eastern China to Hainan province, Hainan locating at the bottom of cold high pressure or locating at the northwest side of the tropical cyclone, temperature dew point difference above 5℃ are helpful to occur the regional O₃ pollution in Hainan province.