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.
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