Abstract: With the rise of river inflow on-line monitoring technology, this paper studies the main technical links of on-line monitoring of inflow into the sea based on the principle and steps of on-line monitoring technology. We suggest a comprehensive and all-round streamlined calculation scheme of rivers runoff into the seas. The method was applied to calculate the example in Liao River inflow. This study provides support for the popularization of on-line monitoring technology of inflow into the sea and the accurate estimation of pollutant discharge from rivers.