The shear lag effect in frame-tube structures can be neglected when the ratio of beam's span to height increases to some degree,and its behaviors comes close to the behaviors of frame structures.The maximum ratio of beam's span to height is obtained by the equal side-sway of two structures.The structure should belong to frame structures when its ratio of beam's span to height exceeds the critical value.Otherwise,it should belong to frame-tube structures.According to the analysis of the various parameters,some influence factors can be obtained,and the process to distinguish the two structural systems is given.It is shown that this method is accurate and convenient enough for use during the preliminary stage of design.