Due to the strong absorption of terahertz(THz) by water, the detection of terahertz spectroscopy for aqueous samples faces significant challenges. To address the issue of high-sensitivity detection of aqueous samples using terahertz spectroscopy, this paper proposes and experimentally verifies a high-resolution frequency-domain spectroscopy system based on terahertz Attenuated Total Reflection(ATR). The system employs an innovative optical heterodyne coherent detection technique,achieving excellent dynamic range and resolution performance. Within the range of 0.3 to 1.2 THz, the peak dynamic range exceeds 100 dB, and the frequency resolution reaches up to 100 MHz. The innovative ATR architecture effectively enhances sensitivity, and in experimental measurements of aqueous solutions with different concentrations of α-lactose, the system has realized direct and accurate quantitative detection of the aqueous samples.