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The principle of maximum conformality (PMC) provides a convenient way for setting the optimal renormalization scales for high-energy processes, which can eliminate the conventional renormalization scale error via an order-by-order manner. At present, we make a detailed PMC analysis on the Higgs decay H → γγ up to three-loop QCD corrections. As an important point of deriving reliable PMC estimation, it is noted that only those {βi}-terms that rightly determine the running behavior of coupling constant via the renormalization group equation should be absorbed into the coupling constant, and those {βi}-terms that pertain to the quark mass renormalization and etc should be kept as a separate. To avoid confusion of separating and absorbing different types of {βi}-terms into the coupling constant, we first transform the decay width in terms of top-quark mass into that of on-shell mass and then apply the PMC scale setting. After applying PMC scale setting, the final estimation is conformal and is scheme-independent and scale-independent. Up to three-loop QCD corrections, we obtain a PMC scale GeV ∼2MH, which is optimal and highly independent of any choice of initial scale. Thus, we obtain a more accurate scale-independent prediction by taking the Higgs mass as the same as that of ATLAS and CMS measurements, i.e., and keV, where the error is caused by the measured Higgs mass, i.e. the Higgs mass MH is taken as GeV for ATLAS and 125.7 ± 0.3 ± 0.3 GeV for CMS, respectively. [ABSTRACT FROM AUTHOR]