The terahertz pulse signal has the characteristics of "fingerprint spectrum" in the frequency domain, which can be used for qualitative analysis of substances. A secondary aspheric TPX plano-convex lens was designed to improve the ability of lens to focus on the terahertz beams, with the help of optical analysis and optimization functions of Zemax software. The terahertz beam shaping optical system was designed by using the plano-convex lens, and the optical system was used in a terahertz time-domain spectroscopy system. The terahertz spectroscopy tests were performed on moxifloxacin hydrochloride and levofloxacin, and the absorption coefficient and refractive index curve in frequency domain were obtained after algorithm processing. The test results show that the refractive index of levofloxacin is higher than that of moxifloxacin hydrochloride in the range of 0.1 THz~3.5 THz band, but the change of the refractive index of moxifloxacin hydrochloride is more gentle than that of levofloxacin. The moxifloxacin hydrochloride has obvious absorption peaks at 1.03 THz, 1.92 THz, 2.58 THz and 2.84 THz, and the levofloxacin has obvious absorption peaks at 1.35 THz, 1.96 THz, 2.52 THz, 2.73 THz.