In order to obtain the high-quality seafood, fish welfare is addressed during culture progress. Healthy, clear, comfortable environment is the key to ensure fish welfare. Production of Atlantic salmon (salmo salar) in recirculating aquaculture systems (RAS) based on fish welfare is being considered as the trend of salmon breeding. Because of the low water-renew rate in RAS, kinds of toxic chemicals such as ammonia, colors are easily accumulated. Ozone as a strong oxidant plays an important role in optimizing water quality in RAS while overdose ozone produced oxidants (OPO) are harmful even fatal to aquatic animals. However there is limited information on how ozone impacts welfare of salmon and the perfect dose of ozone in RAS, and how to prevent the stress of overdose ozone in aquaculture. We proposed to investigate the mechanism of how ozonation influences the welfare of Atlantic salmon by the neuroendocrine response, the physiological change, and the effect at the whole-animal level. Firstly, the acute trail of ozone was carried out to analyze the tolerant and half-lethal dose of ozone in seawater. Based on acute toxic trail, the influence of ozonation, which produces OPO in water and the better hydrochemical-microbial environment for fish in RAS on immunity, cortisol, growth, feeding intake, gill tissue,and body color alteration of salmon is studied. It aims to reveal the mechanism of how ozone acutely/ chronically impacts the welfare of salmon. And this study would provide data on the influence of ozone on the aquatic animals, information to the further mechanical study on how ozone harms aquatic animals and guide of ozone utilization in aquaculture.