The inner compass of the acoustic Doppler current profiler(ADCP)hydrological monitoring system with steel buoys is affected by a ship’s magnetism, which leads to deviation of the measured flow direction. Based on data measured at Xuliujing Hydrological Station during the spring tide and middle tide from 2009 to 2011, the effects of steel buoys on the flow measurement accuracy of an ADCP flow measurement system and the calculation results of discharge and tidal volume were investigated under the conditions when the angle perpendicular to the cross-section was used as the flow direction(without flow direction correction)and when the measured mean flow direction of the verticals on the cross-section was used as the flow direction(with flow direction correction). The results show that, compared with the data measured with the vessel-mounted ADCP, the discharge and tidal volume of flood tides and ebb tides and the sum of tidal volume show small errors and high precision, whether the flow direction is corrected or not. It is concluded that the flow direction of the ADCP system with steel buoys can be uncorrected in the calculation of discharge and tidal volume.