In order to solve the problem that the existing terahertz (THz) characterization system does not fully characterize the sample properties, a wide-spectrum THz system based on gallium selenide (GaSe) crystals is built. Femtosecond pulse laser is used to pump the GaSe crystals, and the generated THz radiation has a broad spectral range of 10 to 20 THz. A Michelson interferometer is built to perform non-coherent characterization of the THz transient electric field. The high-frequency absorption spectra of three kinds of biological samples such as uracil are measured by the system, and the vibration and rotation of biomolecules are analyzed theoretically with density functional theory. Experimental results are in agreement with the theoretical calculations, proving that the system can quickly obtain the high-frequency information of the sample. The system makes up for the lack of information in the high-frequency band of the traditional THz spectral system and lays a solid foundation for building a high-frequency THz time-domain spectral system.