  We consider that the pre-inflation era is radiation-dominated, transiting
smoothly to the inflationary era. We work out in detail the dynamics of
inflaton fluctuations across the phase transition and the proper choices of
initial vacuum states. It is found that this phase transition can suppress
long-wavelength quantum fluctuations of inflaton. This may attribute to the
large-scale CMB anisotropy a lower power than predicted in the standard
$\Lambda$CDM model. In constraining this transitional effect by WMAP anisotropy
data, we use the WMAP best-fit scale-invariant $\Lambda$CDM model with the
density power spectrum replaced by the one found in this work. We find that the
transition occurs at least about 10 e-folds before the comoving scales
comparable to our present horizon size cross the Hubble radius during
inflation.
