Poyang Lake serves as the main feeding and fattening venue for the four major Chinese carps. In recent years, Poyang Lake has experienced a severe drop in water level during the dry season, prolonged dry season, and reduced wetland area. To tackle the problem of water resources, hydrology and water ecology in Poyang Lake, it is suggested to build a hydro-junction project which is proposed to be located at the channel connecting the Poyang Lake and the Yangtze River. Yet the to-be-built Poyang Lake Hydro-junction Project would impede the natural connection between Poyang Lake and the Yangtze River. The migration passage of the four major Chinese carps may be affected. To study the potential impact of the prospective hydro-junction project on the migration of grass carp, analysis was conducted on the hydrodynamic characteristics of the migration channel after the construction of Poyang Lake Hydro-junction Project using two-dimensional and three-dimensional hydraulic models. The swimming ability of grass carp was determined through experiment and literature review. According to the study results, more than 83.74% of the days suitable for fish passage at the entrance to the lake. The hydrodynamic conditions at the entrance and the channel into the Yangtze River had an insignificant impact on the fish migration at the time when the juvenile grass carps entered the lake. Meanwhile, the gate chamber of hydro-junction project could still maintain a high passing efficiency during the peak period of fish migration. While under the designed scheduling mode, the hydrodynamic conditions in front of the hydraulic construction had little effect on fish migration in the years of wet and normal, but the days that were favorable for the fish departure from the lake decreased in dry years caused by almost static velocity in front of the hydraulic construction when the adult grass carps left the lake. The overall hydrodynamic conditions of Poyang Lake could meet the needs of the grass carp migration during the operation period of the Poyang Lake Hydro-junction Project. However, the current design of the fishway of the Poyang Lake Hydro-junction Project suffered the excessive flow velocity in some parts during high and low water level period, which dissatisfied the required passage conditions based on fish swimming ability under the existing scheduling mode. This study was expected to provide a scientific reference for the design and operation of the Poyang Lake Hydro-junction Project from the perspective of river-lake migration of the four major Chinese carps.