A new sandwich sample holder is employed to examine the terahertz (THz) spectrum characteristics of an aqueous solution of bovine serum albumin (BSA) with the aim of exploring the interaction between water and BSA molecules and the effect of the BSA’s molecular vibration frequency on the THz absorption coefficient. In the frequency range of 0.1 to 2.5 THz, the spectral signal strength of the BSA aqueous solution is lower than that of the BSA solid sample; furthermore, the absorption coefficient reduces with the growth of its concentration and is always higher than the absorption coefficient of the solid BSA. The achieved results also reveal that the partial resonance absorption peaks of solution samples and solid samples are the same, signifying that a sandwich sample holder could be effectively employed to detect the BSA aqueous solution. We also utilize GaussianView6.0 software to conduct theoretical simulations on glycine small molecules and find out that their absorption coefficients in the presence of solution and solid conditions also exhibit similarities over a wide frequency range. The research results can be regarded as a solid reference for further exploring the THz absorption characteristics of protein aqueous solution.