  We study the Higgs sector of the secluded $U(1)'$-extended MSSM (sMSSM)
focusing on CP violation. Using the one-loop effective potential that includes
contributions from quarks and squarks in the third generation, we search for
the allowed region under theoretical and experimental constraints. It is found
that the possible region for the electroweak vacuum to exist is quite limited,
depending on the parameters in the model. The masses and couplings of the Higgs
bosons are calculated with/without CP violation. Even at the tree level, CP
violation is possible by complex soft SUSY breaking masses. Similar to the CPX
scenario in the MSSM, the scalar-pseudoscalar mixing enables the lightest Higgs
boson mass to become smaller than the $Z$ boson mass while the coupling with
the $Z$ boson is sufficiently suppressed to avoid the LEP experimental
constraints. However, unlike the CPX scenario, large $\mu$ and $A$ are not
required for the realization of large CP violation. The typical spectrum of the
SUSY particles is thus different. We also investigate the possible upper bound
of the lightest Higgs boson in the case of spontaneous CP violation. The
maximal value of it can reach above 100 GeV with maximal CP-violating phases.
