Based on the three-dimensional homogenization theory, the Characteristic 3D elementary pattern in the masonry was established. The homogenized masonry wall was considered as a vertical plate with four supports. The plate theory was used to research the out-of-plane mechanical properties of masonry panels. Homogenization models with theory of thin plates and theory of medium and heavy plates were given and the calculation formula of deflection of homogenization masonry wall were deduced as well. The mid span deflections of 120 mm, 180 mm, 240 mm and 370 mm masonry walls with both fixed and variable ratio of height to width were analyzed. The applicability of two kinds of plate theories on homogenization masonry walls of different sizes was obtained. This methodology was practical and the masonry theory was improved.