The invention relates to application and a method of fluorine oil as a solvent for reducing the water sensitivity of a liquid phase biological sample for terahertz wave detection, by utilization of the principle that the fluorine oil has very low absorption in THz frequency band, and can replace solvent water with very high absorption around a biological substance, the problem of the water sensitivity of the liquid phase biological sample for terahertz wave detection can be solved, the invention also discloses a method for fixing a to-be-tested substance in the to-be-tested biological sample in a detection device, using the fluorine oil for scouring the sample for replacement of water molecules around the substance, and finally performing THz spectrum detection, the method has the advantages of simple operation, can reduce the signal loss of a THz detection signal passing through the liquid sample, improves the detection sensitivity, and has important significance for the application of THz spectroscopy in the biomedical field.