We study the mixing of the QCD/QCD-like axion and light-flavor mesons π 0 , η, η ′ within the framework of U (3) chiral perturbation theory up to next-to-leading order in this work. The axion-meson mixing formulas are calculated order by order in the U (3) δ-expansion scheme, namely the joint expansions of the momentum, light-quark masses and 1/N C . We provide axion-meson mixing relations in terms of the π 0 -η-η ′ mixing parameters and their masses. The recent lattice simulations on the η-η ′ systems turn out to be able to offer valuable inputs to constrain the unknown low-energy constants. The relation of the mass and decay constant of the axion is then further explored based on our updated calculations. The two-photon couplings of the light-flavor mesons, together with the axion, are also investigated in the U (3) chiral theory up to next-to-leading order in the δ-counting scheme.