There are lots of gravel shoals in upper course of the Yangtze River.Most of these gravel shoals hindered the passing of ships.The formation conditions and the navigation-obstructing characteristics of gravel shoals are discussed which provides more basic theories and information for regulation measurement.Based on the analysis of formation and evolution of gravel shoal,the research deduces the criterions of formation of gravel shoals through the mechanism of sediment incipient.The result shows that the gravel shoal can be formed in a certain range on both of channel slope and channel width.The formula verified by a large amount of field data and experimental data.