Abstract: An eco-composite wall, acting as the main bearing member of a structure, is made up of a concealed frame and eco-composite wall boards. It is extremely complicated to determine the elastic constants of the wall because of the special construction and different materials used in the wall. It is also difficult to calculate the lateral stiffness of the wall accurately. Based on the previous experimental study, aimed to the special construction of the wall and combined the theory of composite material mechanics, a single-layer composite material model of bi-directional fiber for the wall in the elastic stage is established. The practical design formulas for the elastic modulus and shear modulus are given, which provide the necessary parameters for the practical design formula for elastic rigidity. The lateral stiffness of an equivalent elastic composite slab is deduced by use of elastic mechanics theory. The comparative analysis between the theoretical calculation and test results shows that the composite material mechanical model is practical and effective for calculating the lateral stiffness of the wall.