  We developed a multi-region radiation model for the evolution of flux and
spectral index with time. In this model, each perturbation component in the jet
produces an independent flare. The model can be used to study the decomposition
of microvariability, the structural scale of the perturbed components, and the
physical parameters of the acceleration processes. Based on the shock
acceleration model in relativistic jet, the influence of acceleration
parameters on multiband flare parameters is calculated. We present the results
of multiband optical microvariability of the blazar BL Lacertae observed
performed during 89 nights in the period from 2009 to 2021, and use them as a
sample for model fitting. The results show that both the amplitude and duration
of flares decomposed from the microvariability light curves confirm a lognormal
distribution. The time delays between the optical bands follow the normal
distribution and amount to several minutes, that corroborates with both
predictions from the theoretical model and the calculation of the discrete
correlation function (DCF). Using the spectral index evolution and the
simultaneous fitting of the multiband variability curves, we obtain the
acceleration and radiation parameters to constrain and distinguish the origins
of different flares. Based on the flare decomposition, we can well reproduce
the time-domain evolution trends of the optical variation and energy spectrum,
and explain the various redder-when-brighter (RWB) and/or bluer-when-brighter
(BWB) behavior.
