  Considering that the universe is filled with the nonrelativistic matter and
dark energy and each component is respectively satisfied with its conservation
condition in the absence of their interaction, we give the change rate of the
fractional density and the density of dark energy from the conservation
condition. It is clear that the fractional density of dark energy will
monotonously increase and gradually become the dominating contribution to the
universe as the redshift becomes low. Combining the evolutional trend of the
state equation of dark energy and the change rate of the density of dark energy
we find that the density of dark energy will decrease up to a minimum and
whereafter it will increase again as the redshift becomes low. This can be
regarded as the phase transition of dark energy from the quintessence phase to
the phantom phase.
