Abstract: The coupled field distribution of high power microwave in buildings is calculated by physical optics method. With the transmittivity of the wall and window, the approximate field distribution on the inner surface of wall can be calculated. Substituting the surface transmitted field distribution of the wall into the vector diffraction formula, we can get the transmitted field in the building. With the reflectivity of the floor, the approximate reflection field on the floor's surface can be calculated. Substituting the surface reflected field distribution of the floor into the vector diffraction formula, we can get the reflected field in the building. The superposition field is obtained by vector addition of transmission field and reflection field. The field distribution result of this method and that of the finite-difference time-domain method are in good agreement. The advantages of physical optics method lie in its clear physical image, small amount of computation and fast computation speed.The method is suitable for the calculation of coupled field distribution in large buildings.