The ground-level ozone (O₃) concentration is continuously increasing in middle areas of China during recent years.Volatile organic compounds (VOCs) are important precursors in photochemical production of ozone,which is of great significance for O₃ prevention and control.In order to clarify the characteristics of ambient VOCs and their role in ozone formation in China's middle areas,we conducted online VOCs observation in Changsha from May to October in 2021,monitoring 116 VOC species in total.The mean value of mixing ratios for measured VOCs was (23.09±9.97)×10^-9.Monthly average concentration of VOCs in Changsha showed a "U" shape,with the lowest in July and the highest in October,while the diurnal variation of VOCs concentration showed a bimodal pattern,indicating the influence of human activities.OVOCs were the most abundant component of VOCs,followed by alkane and halo carbon.However,OVOCs and aromatics were the two largest contributors to ozone formation potential (OFP),with a cumulative contribution of 68.3％.Propionaldehyde,acetaldehyde,m／p-xylene,ethylene and toluene were the most important VOCs species to OFP.The observation-based model (OBM) showed that O₃ formation in Changsha was in a transition regime in May,August,September and in a VOCs-limited regime from June to July,while in a NO_x-limited regime in October.For the anthropogenic VOCs,ALDP,OLEP and PARP should be given priority in emission control measures.