Spectroscopy has become an effective detection tool in fields such as environmental monitoring, food safety, and public health. Terahertz time domain spectroscopy (THz-TDS) is notable for its label-free and nondestructive evaluation capabilities, as well as its ability to extract material properties such as dielectric constants and absorption coefficients. However, the intrinsic damping of molecular materials significantly limits the sensitivity of THz-TDS in detecting trace molecules. In our study, we utilized a THz-TDS system to measure the terahertz spectra of water vapor molecules at varying interaction distances d. By employing synthetic complex-frequency waves (CFW) excitation, virtual gain was introduced to counterbalance the intrinsic damping of water vapor molecules, thereby enhancing the sensitivity of terahertz detection. Experimental results show that applying synthetic CFW excitation to spectral responses containing phase information notably improves the detection sensitivity of water vapor molecules absorption features in the terahertz range at two distinct interaction distances d. Synthetic CFW excitation amplifies molecular vibrational fingerprint signals within the terahertz range, expanding the potential applications of THz-TDS in fields such as molecular identification, pharmaceutical analysis, and carrier dynamics in semiconductors.