Based on the torsion-bending theory of thin-walled girder,a calculating method of continu?ous girder on elastic brace and the derivation of three-dimensional straight member stiffness matrix, are presented to analyze spatially the straight concrete beams. Namely,the diaphragm is regarded as e?lastic braces,and the beam as continuous girder on elastic brace. Both of the beam’s internal force un?der the eccentric load and the correponding internal force of diaphragms are calculated by this method.In the end of this paper, a comparison is made between the proposed method and the traditional method by a numerical example.