  It is shown in this paper that decoherence-free subspace (DFS) of practical
multi-photon polarization can not avoid the exponential decoherence even in the
same extra-environment if the photons are frequency-anticorrelated. The reason
lies in that the condition of collective decoherence is not satisfied in this
case. As an example, the evolution of biphoton's decoherence-free state is
given. Possible solution for feasible multi-photon's DFS state is also given.
