  We present a self-consistent approach to deal with the pairing-fluctuation
effects in quasi-two-dimensional superconducting systems. Besides the Cooper
pairs in the Bose-Einstein condensate, there are pairs occupying the excited
states, which results in the predominant fluctuations. The low-lying excited
states are the collective modes. On the basis of ladder-diagram approximation,
we treat the single particles and the pairs on an equal-footing manner. The
Green's function of single particles is obtained as an analytic solution to a
cubic equation. The bosonic degrees of freedom are relevant to the pseudogap
physics in the high-T_c cuprates. The superconducting order parameter and the
transition temperature are substantially reduced from the values of the
mean-field theory. The calculated phase boundary of superconducting state can
reasonably describe the experiment data for cuprates.
