Abstract: A simple and accurate method to analyse the framed-tube structures under torsion as well as a 15-story framed-tube plexiglass model test is presented in this paper. The model is designed with widely spaced columns on the ground floor.The torsional angles of the structure and the strains in the 1st.floor and the 2nd.floor are measured.The distribution of internal shear in these columns is obtained. Based on the results of the model test,a three diamensional framed-tube structure can be replaced by an equivalent plane frame,and the fictions horizontal members with specially prescribed properties are added at the corners of the structure.The action of torsion can be represented by a pair of horizontal shear forces in the frame plane. The results from the theoretical analysis have good agreement with those from the model test by us and from another model test in reference by others.Finally,some principle structural parameters are presented and are discussed.