In view of masonry structures with rigid floor slab, seismic behavior of R. C. stairs and influence of stairs on the stiffness of lateral walls of stair well was analyzed, calculation formulae of the seismic internal forces of stairs were deduced, calculation method of the seismic shear forces considering the seismic effect of stairs on main structure were given out. Analysis and comparison on engineering ensamples were carried out. The results show K-type bracing function of step slabs to main structure is evident. Considering the bracing function, shear deformation of the floor layer and shear forces in the seismic walls are decreased, internal forces in R. C. stairs are increased. Seismic design recommendation of step slab to decrease the seismic forces was presented.