The key point to measure a sample and to carry out its absorption spectrum is that the whole test process is completed
in very stable situations. Although the distributed setup of THz-TDS is flexible to implement kinds of experiments,
slightly displacements of mirrors may occur in the system. A typical kind of disturbance exists in the system due to shifts
of mirrors and accumulation of static charges in THz switch or detector, particularly in a long term test. The resulting of
the disturbance is that a deviation is involved in the THz pulse. Furthermore, due to the sample and reference can't be
tested simultaneously, the sample's absorption spectrum maybe goes down to negative values at some frequencies. A
recovery algorithm is presented to overcome the disturbance. Some examples validate the effects of the algorithm.