A new interpretation on the low frequency absorption band in Hg1-xCdxTe is presented.By the calculation of the vibrational spectra of CdTe (or HgTe) diatomic chain with one substitutional impurity Hg (or Cd), and the discussion of the characteristic of vibrational modes in the mixed crystal Hg1-xCdxTe, ive show that there exist acoustic phonons, the frequency of which doesn't approach zero at k→0. These acoustic phonons can absorb the photons in far infra red range and cause the low frequency absorption band. The characteristics of low frequency absorption band reported so far can thus be interpreted. We also predict that there may exist similar low frequency absorption band in the other mixed crystals, the optical phonons of which have "two-mode" behaviour.