  The optical/ultraviolet (UV) variability of quasars has been discovered to be
correlated with other quasar properties, such as luminosity, black hole mass
and rest-frame wavelength. However,the origin of variability has been a puzzle
so far. In this work, we upgrade the accretion disc model (Li & Cao 2008),
which assumed the variability is caused by the change of global mass accretion
rate, by constraining the disc size to match the viscous timescale of accretion
disc to the variability timescale observed and by including the
irradiation/X-ray reprocessing to make the emitted spectrum become steeper. We
find this hybrid model can reproduce the observed bluer-when-brighter trend
quite well, which is used to validate the theoretical model by several works
recently. The traditional correlation between the variability amplitude and
rest-frame wavelength can also be well fitted by our model. In addition, a weak
positive correlation between variability amplitude and black hole mass is
present, qualitatively consistent with recent observations.
