This paper presents a detailed theoretical analysis of the phenomenon of the second optical harmonic generation by two surfaces of a transparent isotropic glass plate. The dependence of intensity of the second harmonic wave generated by thermally-poled soda-lime glass plate on the incidence angle of fundamental wave (Maker fringes) has been measured. The obtained results of the modeling and the experiment were compared qualitatively and quantitatively. The comparison showed that thermal poling significantly increased quadratic nonlinear susceptibility χ of the subsurface region of the poled glass. We estimated this relative increase and obtained the following: χzzz, χzxx, χxzx increased about 6, 9 and 19 times respectively.