In plain river network

joint nodes can not be ignored for exchange of water flow and pollutants from different watercourses. Response relation between outlet sections and inlet sections is an important technical parameter for water quality simulation. Aiming at the response relation research of crossed joint nodes with two inlet sections and two outlet sections

this paper builds 2-D hydraulic and water quality mathematical models and validates them by physical models. The results show that pollutant concentration differs greatly for different outlet sections. Based on the work above

a new expression of response relation and calculating equations of response coefficients are put forward. By comparing the response coefficients with those inferred from even mixing hypothesis mainly used in threestep method

obvious errors caused by even mixing hypothesis are disclosed.This study helps to improve the simulation method of plain river network water quality.