A reference-free spectroscopic analysis method for terahertz (THz) transmission and reflection measurement is presented. The absorption features of weakly polarized materials (RDX, 2,4-DNT and HMX) are extracted without the reference signal. For transmission spectroscopy, the absorption peaks can be located from the negative first-order derivative of the sample signal phase divided by the frequency. The absorption signatures of the reflection spectroscopy of the materials are extracted directly from the second-order derivative of the phase of the sample beam with respect to frequency. For reflection spectroscopy, the absorption signatures of the materials are extracted directly from the second-order derivative of the phase of the sample beam with respect to frequency. This technique provides a straightforward and fast solution to solve the phase-retrieval problem in reflection fime-domain spectroscopy (RTDS) and eliminates the “misplacement error”.