The electro-optic (EO) technique used in the terahertz spectral technology is described. Response functions—which are used to describe the EO detecting efficiency—of five types of cubic EO crystals are calculated. Based on these results, the choice of different crystal thicknesses is discussed. In the EO detecting procedure, certain factors, such as reflection, dispersion, absorption and velocity mismatch, may distort the temporal profile of the electric field being detected; therefore, these factors are calculated. The relationship between the field distortion and the bunch duration as well as the crystal thickness is considered.