Abstract: A kinematic differential equation and criterion to discriminate sliding from joggling state were developed by virtue of an interstorey shearing model with multi-degree of freedom considered for those multi-storey masonry structures which are built on an isolated frictional base and under the action of earthquake in both horizontal and vertical directions, which a numerical analysis is made to the influence of vertical action on earthquake-isolated structures. The results show that the vertical action enables the lower storeys especially the ground or 1st storey to displace. Thus, the interstorey shear and displacing acceleration will increase and go on further with the higher peak value of vertical acceleration when the earthquake intensity is increasing. It is also possible that the sliding displacement amplitude will increase then. So, it is highly recommended that the influence of vertical earthquake action on such structures shall be taken into account in high-intensity earthquake regions.