Water has the strong absorption of terahertz (THz) wave, so it always a difficult problem to study the characteristics of aqueous samples by THz technology. In this paper, THz waves with high field strength are obtained at the position of sample by using photoconductive antenna working in high-gain mode and horn shaped graded parallel plate waveguide in THz time-domain spectrum system, and the THz spectrum of α-lactose solution in a range of 0.1－1.5 THz is directly detected. Furthermore, the absorption spectrum of α-lactose single molecule model in water environment is simulated by the density functional theory, and the simulation results are in good agreement with the experimental results. This work has important reference value for directly detecting the spectral characteristics of water samples in THz band.