Nanchang Honggu Tunnel is a longest inland river tunnel in China. The immersed section of Honngu Tunnel is 1 329 m long and includes 12 segments. The axial line accuracy of segment installation is the key. The main influencing factors of tunnel segment installation accuracy are calculated, and the axial line deviation rectification of tunnel segment is carried out by using horizontal adjusting system. The comparison between measured data and theoretical calculation results shows that the abovementioned axial line deviation rectification method is effective. The study results can provide reference for similar projects in the future.