Abstract: This paper points out the slabs flexure stiffness of a slab-column structure subjected to horizontal forces changes along its span. The flexure stiffness of slab intersected with column is smaller than that of midspan slab. According to the principle which slabs stiffness equals to equivalent beams stiffness, the formula of calculating width for equivalent beam with equal width is given out by using finite element analysis. The calculated result is basically in accordance with that of the test.