In accretion systems, outflows may have significant influence on luminosity fluctuations. In this paper, following Lyubarskii’s general scheme, we revisit the power spectral density of luminosity fluctuations by taking into account the role of outflows. Our analysis is based on the assumption that the coupling between the local outflow and inflow is weak for accretion-rate fluctuations. We find that, for inflow mass accretion rate , the power spectrum of the flicker-noise component will present a power-law distribution p(f) ∝f−(1 + 4s/3) for advection-dominated flows. We also obtain descriptions of p(f) for both standard thin discs and neutrino-cooled discs, which show that the power-law index of a neutrino-cooled disc is generally larger than that of a photon-cooled disc. Furthermore, the relationship obtained between p(f) and s indicates the possibility of evaluating the strength of outflows through the power spectrum in X-ray binaries and gamma-ray bursts. In addition, we discuss the possible influence of outflow–inflow coupling on our results.