Based on classical polarization theory, the change of refractive index of silicon is analyzed when direct-current (dc) electric field, alternating-current (AC) modulation electric field, and optical field exist simultaneously, and the physical nature of the electric-field-induced linear electro-optic effect is theoretically indicated. Near-intrinsic silicon is constructed into a metal-insulator-semiconductor sample, and a transverse electro-optic modulation system composed of the modified Senarmont compensator is applied. The obvious linear electro-optic modulation signal is observed in the space charge region of silicon, and the half-wave voltage of the modulation system less than 170 V is estimated. The electric-field-induced linear electro-optic effect caused by the built-in electric field in silicon is experimentally demonstrated. In addition, the secondary electro-optic modulation signal generated by Kerr effect, and electric signal which changes with the cosine function of double azimuth of the linear polarizaed light and is induced by electric-field-induced optical rectification, are measured. These results are in good accordance with the classical polarization theory, and demonstrate the electric-field-induced linear electro-optic effect indirectly.