A Lumped-Mass method is presented for the first time to analyze the tension force subjected by flexible mattress and its configuration.The method takes account of the elastic extension,non-uniform mass distribution of the mattress and the acting force due to flowing water.Comparing the calculation result by this method and that based on the catenary theory,we know that the Lumped-Mass method is more applicable for analyzing the flexible mattress in real fluvial environment than the commonly used catenary theory.