Abstract: The construction of reservoirs promotes the development of fishery, but discharge from high dam leads to the generation of total dissolved gas (TDG) supersaturation, which is detrimental to the fish in downstream reservoirs and may cause gas bubble disease (GBD). To investigate the effect of TDG supersaturation on fish in the Xiangjiaba Reservoir on the Jinshajiang River, a laterally averaged dynamic model for TDG transport was established, and the threshold of safe depth for the control section was investigated. Then the impact of TDG supersaturation on wild freshwater fish and cage-culture fish was studied based on habits and net cage depth. The results show that: the reservoir provides enough vertical space for wild freshwater fish to swim and acquire hydrostatic compensation to avoid GBD, and the risk of fish suffering from TDG supersaturation depends on the fish habits and the ability of utilizing hydrostatic compensation. For the limit of net cage depth, cage-culture fish have no enough vertical space to swim to safety depth. In order to avoid fish suffering from GBD, the net cage depth should be deeper than the threshold of safe depth, and the fish should be kept swimming under the threshold of safe depth during the period of TDG supersaturation. The reservoir between cascade hydropower stations is safer than the reach downstream of the last cascade hydropower station, and it is necessary to set a most strict TDG limitation standard for the last cascade hydropower station.