According to the stress characteristics of masonryRC wall composite structures, the paper puts forward the calculation formula of the optimal number of RC walls under the action of earthquake. In the formula, the main influence factors of parameter of amount of RC walls are: the characteristic site period，the seismic fortification intensity，the elastic allowable deformation value, the stiffness characteristic coefficient of composite structures, the compensationcoefficient of the nonbearing walls on natural period of vibration,etc. According to the main influence factor, the stiffness characteristic coefficient λ, this study gets the value tables of parameter of amount of RC walls in the calculation formula of the optimal quantity of concrete wall under various antiearthquake grades，ground categories and design groups. A numerical example shows this method is simple and practical.It can be used for the initial design stage of this kind of composite structures in seismic regions.