Abstract: The flow volume of the river into the sea is a key parameter affecting the fluxes of pollutants discharged from the river to the sea, and is as well a technical difficulty in the monitoring of pollutants entering the sea. In view of the problem that the total amount of pollutants in the river is unknown, this paper focuses on the two key technologies affecting the monitoring the runoff flux into the sea, that is, the monitoring section setting and the flow monitoring techniques. We analyzed the practicability of the setting of the monitoring sections in different channel segments through comparing with the geographical and the hydrodynamic characteristics of the estuarine river section, transition section and tidal current section. In addition, this paper investigated the four methods including representative points, representative layers, representative vertical and combined flow which are currently used of monitoring the flow pattern in the transition section of the river into the sea. At last, taking the Liaohe River under the conditions the influence of the gate dike, the transition section, the complex terrain and the shallow water depth as a case, we validated the method based on the monitoring data in the marine environment online monitoring test sites during the classic wet season in 2016.The result suggests that this method can accurately reflect the flow of rivers into the sea, and then can provide the technology support for monitoring the pollutions from the rivers into the sea in China.