The terahertz waves exhibit spectral fingerprint characteristics and penetrability for nonpolar substances, which have huge potential applications in the identification of hazardous substances hidden in mail. Currently, the commonly used terahertz optical-constant-extraction algorithms have the disadvantages of being complicated and requiring lengthy computations as well as samples with known thicknesses. In this study, we propose a method of directly calculating the characteristic absorption frequencies based on the amplitudes of the terahertz transmission coefficients to alleviate the shortcomings associated with the conventional algorithms and satisfy the rapid-processing requirement of real-time mail detection. The calculation procedure is considerably simplified by using the proposed method because the calculation only needs to be conducted once. To further improve the processing speed, we propose data processing methods that use abnormal data filtering and low-pass filtering instead of requiring multiple measurements and average filtering. Therefore, all the data measurements can be completed once. Furthermore, the characteristic absorption frequencies of the sucrose and benzophenone samples obtained using the proposed method are consistent with those obtained using the conventional method.