Abstract: The orthotropic character of D3=D1D21/2 of a cast-in-situ concrete hollow slab is deduced.Based on the differential equation of the deformed surface of the orthotropic plate, one principal direction size is kept invariably, then another principal direction size is transformed linearly, maintains elasticity coefficient is kept the same as that of the first principal direction E=E1, Poisson ratio =121/2, thus the original orthogonal plate can be equivalent to an isotropic plate. Results are obtained through analysis: the deflection of the equivalent isotropic plate is the same as that of the original orthotropic plate at the corresponding point, whose internal forces have the simple relations Mx=Mx1、My=k1/2My1、Mxy=k1/4Mx1y1.