Abstract: River auto-selection is an essential part of automatic thematic map generalization.When ac-complishing the selection,it is critical to reasonably evaluate each river’s importance in the globalstructure.A grading system is needed to quantify the importance of each river.In this system,theimportance of a river differs at different levels.Although several grading systems(Horton,Strahler,etc.)are already in use,each with its strengths,still they cannot completely meet the requirementsfor river auto-selection,therefore,a basin-based grading system of river classification is proposed inthis study.Each river’s importance in partial river networks is considered and selected accordingly.This system can make the density of the selected rivers as consistent as the entire network.