  In this Letter, we study analytically the evolutions of the flat
Friedmann-Lemaitre-Robertson-Walker (FLRW) universe and its linear
perturbations in the framework of {\em the dressed metric approach} in loop
quantum cosmology (LQC). Assuming that the evolution of the background is
dominated by the kinetic energy of the inflaton at the quantum bounce, we find
that both evolutions of the background and its perturbations are independent of
the inflationary potentials during the pre-inflationary phase. During this
period the effective potentials of the perturbations can be well approximated
by a P\"oschl-Teller (PT) potential, from which we find analytically the mode
functions and then calculate the corresponding Bogoliubov coefficients at the
onset of the slow-roll inflation, valid for any inflationary model with a
single scalar field. Imposing the Bunch-Davies (BD) vacuum in the contracting
phase prior to the bounce when the modes are all inside the Hubble horizon, we
show that particles are generically created due to the pre-inflation dynamics.
Matching them to those obtained in the slow-roll inflationary phase, we
investigate the effects of the pre-inflation dynamics on the scalar and tensor
power spectra and find features that can be tested by current and forthcoming
observations. In particular, to be consistent with the Planck 2015 data, we
find that the universe must have expanded at least $141$ e-folds since the
bounce.
