  We investigate the ground state and finite-temperature properties of the
spin-1/2 Heisenberg antiferromagnet on an infinite octa-kagome lattice by
utilizing state-of-the-art tensor network-based numerical methods. It is shown
that the ground state has a vanishing local magnetization and possesses a
$1/2$-magnetization plateau with up-down-up-up spin configuration. A quantum
phase transition at the critical coupling ratio $J_{d}/J_{t}=0.6$ is found.
When $0<J_{d}/J_{t}<0.6$, the system is in a valence bond state, where an
obvious zero-magnetization plateau is observed, implying a gapful spin
excitation; when $J_{d}/J_{t}>0.6$, the system exhibits a gapless excitation,
in which the dimer-dimer correlation is found decaying in a power law, while
the spin-spin and chiral-chiral correlation functions decay exponentially. At
the isotropic point ($J_{d}/J_{t}=1$), we unveil that at low temperature ($T$)
the specific heat depends linearly on $T$, and the susceptibility tends to a
constant for $T\rightarrow 0$, giving rise to a Wilson ratio around unity,
implying that the system under interest is a fermionic algebraic quantum spin
liquid.
