Abstract: In order to master the hydraulic characteristics of semi-closed air cushion surge chamber(SACSC),a hydraulic calculation mathematical model of SACSC composed of explicit equations, easy to programming and suitable for both open and air cushion surge chamber, is derived and established by analyzing the common characteristics of all kinds of surge chambers.In the study, we improve the basic assumptions of gas thermodynamic process of existing models, unify the equation form and improve the discrete accuracy.On this basis, through the research and analysis of the main calculation parameters, large fluctuation hydraulic performance, water level fluctuation stability performance, structure and operation control, the hydraulic characteristics of SACSC are systematically discussed.The research shows that SACSC can operate adaptively, has good performance, simple structure and convenient reformation potential, possessing the advantages of open surge chamber and air cushion surge chamber.