The requirements of water level and flow velocity of segment floating transportation and sinking of immersed tunnel are high. Due to seasonal rain, the water level and flow velocity in Ganjiang River vary greatly and there are few hydrological conditions suited to tunnel segment floating transportation and sinking. The boundary conditions of hydrology are obtained by calculating water resistance and towing force. The water level and flow velocity of key points and control crosssections are monitored by using acoustic Doppler Current Profilers (ADCP), and they are taken as the basis of hydrological forecasting. The water level and flow velocity are predicted by relevant water level method and compound flow rate method; and flow field distribution is calculated by numerical model based on predicted results. The field measured results show that the error of predicted water level and flow velocity are ±30 cm and ±0.15 m/s respectively; the hydrological prediction technologies can meet the accuracy requirements, and can guarantee the safety of segment floating transportation and sinking of Honggu Tunnel.