Crude oil emulsions have an important influence on the long-distance transportation of crude oil. The oil and water state of the emulsion and the interaction mechanism require new theories and new methods. The terahertz spectrum ( THz- TDS) response of

crude oil emulsion is studied based on the effective medium theory. The THz-TDS of emulsions with water content of 0 ～ 28 % are obtained by using the THz-TDS system. The optical parameters such as absorption coefficient and dielectric constant are calculated by

Fourier transform. The optical parameters increase with the increase of water content of emulsions. The actual dielectric constant is consistent with the dielectric constant calculated by Bruggeman's theory，and the maximum error rate is less than 3 % . The results show that when the

water content is small，the water is dispersed in the crude oil，the distance between the particles is large，and the interaction force between the particles is small. As the water content increases，the water phase and the oil phase are mixed and distributed，the distance between the

particles will become smaller，and the inter- action force of the particles will become larger.Therefore，the effective medium theory is significant to investigate the THz spectrum of oil-water emulsions.