  In this work, we investigate the dark sector of a supersymmetric axion model,
consisting of the late-decaying gravitino/axino dark matter and axion dark
radiation. In the early universe, the decay of the scalar superpartner of the
axion (saxion) will produce a large amount of entropy. The additional entropy
can not only dilute the relic density of the gravitino/axino dark matter to
avoid overclosing the universe but also relax the constraint on the reheating
temperature $T_{R}$ after inflation. Meanwhile, the axion dark radiation from
the saxion decay will increase the effective number of neutrino species $N_{\rm
eff}$, which can help to reduce the cosmological Hubble tension. In the late
universe, the decay of long-lived gravitino/axino dark matter produces the
axions with MeV-scale kinetic energy. We study the potential of searching for
such energetic axions through the inverse Primakoff process $a+A \to \gamma+ A$
in the neutrino experiments, such as Hyper-Kamiokande.
