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 to carrying out research for controlling O3 prevention and control. In order to understand characteristics and role in ozone formation of ambient VOCs in middle areas, we conducted online VOCs observation in Changsha City from May to October in 2021, monitoring 117 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 was 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 NOx-limited regime in October. For the anthropogenic VOCs, ALDP, OLEP and PARP should be given priority in emission control measures.