Abstract: For open channel constant nonuniform flow which has local holdback,the research considering the change of hydraulic grade line after the loss of local water head is less at present,and the expression of the loss of local water head is not unified either.This fact is not convenient for defining coefficient of the loss of local water head in unison,neither convenient for considering qualitative analysis of the variation regularity of hydraulic grade line of open channel constant nonuniform flow after the loss of local water head.By way of the test for hydraulic grade line on the open channel of flat gradient and the research of numerical calculation,we found when the flow passed the local barricade,hydraulic grade line in Area a of the open channel of flat gradient appeared into a descending locally Type a₁ dammed hydraulic grade line.