Abstract Terahertz time domain spectroscopy (THz-TDS) is used to investigate the optical properties of nematic liquid crystals E7 and 5CB under the external magnetic field, electric field and optical field in the THz range. It is systematically summarized that the optical property changes of nematic liquid crystals under different external fields. It is experimentally observed that a negative magneto refractive index change under the magnetic field and a positive refractive index change under the electric field. The maximum values of magneto and electric refractive index changes reach - 0.087 and 0.051, respectively. The physical mechanism of photorefractive effect is the same as electric refractive effect, and the maximum value of photorefractive coefficient is 0.015 under the power of 532 nm laser with 7.961 W/cm² output. These research results are significant for the key THz devices such as tunable phase shifters, filters and the spatial light modulator in THz technology and applications.