Abstract: Earthquake damages of a great deae of single-story mill buildings shoh such a rule:while the side-sway and torsion are appearing in the vibration of the bilding,the horizontal deformation of the roofing in both transverse anb longitudinal direction are produced.According to the surveying datas of the dynamic experiment and the horizontal defomation curve of the roof slab of several existing single-story mill buildings, the approximate value of the natura horizontal stiffness of the roof slab is obtained. In this paper,appropriate mechanical modelwith consideration of torsion and roof deformation and effective stiffness of the brick masonry wall is set up,and the "Series-parallel multiple lumped-mass system" is taken for the three-dimensional seismic analysis of the spatial structure subject to the two-way ground motion.The calculating results are approximately close to the rules of the earthquake damage of the damaged single-story mill buildings in tangshan earthquake.