According to the discharge data measured by ADCP and water quality data acquired by online monitoring of the Chendong River in 2021, which is an important channel into the western Taihu Basin, the annual variation characteristics of discharge, mass concentration and influx of pollutant of the river were analyzed. Also, the effects of discharge and water quality sampling frequency on the calculation of pollutant influx were compared. The results show that the annual average discharge of the Chendong River was 73.66m³/s in 2021, and the annual runoff was 2.329 billion m³. The daily discharge fluctuated violently during 2021, with a peak in July, and its change process was significantly affected by precipitation. The mass concentration of CODMn and TP also fluctuated greatly during 2021, with the annual cumulative influx being 10357t and 352t, respectively. The accuracy of pollutant influx calculation based on fixed ADCP discharge measurement data was about 39.5% higher than that based on hydrological patrol survey data. The difference was mainly reflected from June to August. If it is necessary to control the relative error of the calculation of the annual cumulative influx of the two pollutants within 5%, the discharge sampling frequency should be at least 2 days. If the relative error needed to be controlled within 10%, the discharge sampling frequency should be at least 10 days. While during the wet season (from June to September) and the dry season (from December to February), the discharge sampling frequency should be increased to 2 days. Compared with the discharge sampling frequency, the annual cumulative influx of the two pollutants were relatively less sensitive to change of water quality sampling frequency. When the water quality sampling frequency was reduced from 8 hours to 30 days, the relative error was still within 5%.