Si (111) electrode has been widely used in electrochemical and photoelectrochemical studies. The potential dependent measurements of the second harmonic generation (SHG) were performed to study Si (111) electrolyte interface. At diﬀerent azimuthal angles of the Si (111) and under diﬀerent polarization combinations, the curve of the intensity of SHG with extern potential have different form of line or parabola. A quantitative analysis showed that this differences of the potential-dependence can be explained by the isotropic and anisotropic contribution of the Si (111) electrode. The change in isotropic and anisotropic contribution of the Si (111) electrode may be attributed to the increase in doping concentration of Si (111) electrodes.