Plate stiffener is the most basic form of open stiffener， and its local instability is the main failure mode. In order to study the local stability of plate stiffener， two groups of Q420 open flat plate stiffened panel specimens with different thicknesses and heights were designed for axial compression tests， and the corresponding finite element model was established. The simplified calculation formulas for the local stability of the plate stiffener and trilateral simply supported plate were obtained. The experimental and analytical results show that： when the width-to-thickness ratio of plate stiffener is less than 16， the simultaneous buckling failure of plate stiffener and stiffened plate occurs， on the contrary， the local buckling failure of plate stiffener occurs； With the increase of the width-to-thickness ratio of plate stiffener， the ultimate average stress decreases gradually， and the instability deformation of the specimen becomes more obvious.When the relative width-to-thickness ratio exceeds 1.08， the fitted cubic polynomial fitting curve of the plate stiffener in the member is higher than that of other code curves； When the relative width-to-thickness ratio exceeds 1.32， the fitted cubic polynomial fitting curve of trilateral simply supported plate is higher than that of Euler curve， and it is higher than the curves in Chinese steel bridge code and Japanese code in the whole range of relative width-to-thickness ratio. The fitting formula of plate stiffener in the member can accurately calculate the bearing capacity of the actual specimen， and the fitting formula of a trilateral simply supported plate is safer. It is recommended to use the fitting formula of a trilateral simply supported plate for design.