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A cosmological aspect of QCD axion is revisited. The QCD axion acquires the potential through the non-perturbative effect around the QCD phase transition. During this period, the mass of the QCD axion depends on the cosmic temperature. Focusing on the impact of the temperature-dependent axion mass, we have analysed the evolution of the density fluctuation of the QCD axion together with the primordial adiabatic fluctuations, including the scalar metric fluctuation and the density fluctuation of the background radiation component. As a result of the linear perturbation analysis, I will show that the fluctuation of the QCD axion is enhanced model-independently. This result opens up a new possibility for the formation of oscillons followed by an enhanced production of the axion clumps/miniclusters with the pre-inflationary Peccei-Quinn symmetry breaking scenario. This talk is based on arXiv:2111.05785.