In order to calculats the ultimate load of a Waffle_slab structure with the method of plastic analysis,this paper supposes the structure as a virtual isotropic plate of structural orthogonal anisotropic plat es.The formulations of the ultimate moments distribution between the virtual plate and the virtual beams are deduced.The virtual work of the internal forces can be figured out easily under the given failure mechanism models,and then the ultimate load of the structure can be determined according to the virtual work principle.