  We present photometric and spectroscopic observations and analysis of the
type IIb supernova (SN) SN 2019tua, which exhibits multiple bumps in its
declining light curves between 40 and 65 days after discovery. SN 2019tua shows
a time to peak of about 25 days similar to other type IIb SNe. Our observations
indicate a decrease in its brightness of about 1 magnitude in the 60 days after
the peak. At about days 50, and 60, its multiband light curves exhibit bumpy
behavior. The complex luminosity evolution of SN 2019tua could not be well
modeled with a single currently popular energy source model, e.g., radioactive
decay of $^{56}$Ni, magnetar, interaction between the ejecta and a
circumstellar shell. Even though the magnetar model has a smaller $\chi^2 /
\text{dof}$ value, the complex changes in SN 2019tua's brightness suggest that
more than one physical process might be involved. We propose a hybrid CSM
interaction plus $^{56}$Ni model to explain the bolometric light curve (LC) of
SN 2019tua. The fitting results show that the ejecta mass $M_{\rm ej} \approx
2.4~M_\odot$, the total CSM mass $M_{\rm CSM} \approx 1.0~M_\odot$, and the
$^{56}$Ni mass $M_{\rm Ni} \approx 0.4~M_\odot$. The total kinetic energy of
the ejecta is $E_k\approx 0.5 \times 10^{51}\rm~erg$. Pre-existing multiple
shells suggest that the progenitor of SN 2019tua experienced mass ejections
within approximately $\sim6 - 44$ years prior to the explosion.
