Yttrium-aluminum garnet/yttria-stabilized zirconia (YAG/YSZ) double-ceramic-layer thermal barrier coating (TBC) with NiCoCrAlY bond coating was deposited by atmospheric plasma spraying (APS) on Inconel 738 alloy substrate, and laser modification was carried out on the surface of YAG ceramic coating to improve the high- temperature oxidation resistance of TBC. Then, isothermal oxidation tests were designed to study the high-temperature oxidation and thermally grown oxide (TGO) growth behaviors of the as-sprayed (AS) and the laser modified (LM) YAG/YSZ TBC. Results show that TGO thickness and structure of both AS-TBC and LM-TBC exhibit the similar evolution trend, TGO thickness increases with the increasing of isothermal oxidation time, and TGO structure develops from single Al₂O₃ layer to the double-layer structure of upper mixed oxides and lower Al₂O₃. Because of the crucial inhibition effect of the laser modification on oxygen permeation, the appearance time of mixed oxides in LM-TBC is postponed, and the total TGO thickness of LM-TBC is reduced at the middle and final oxidation stages. The Al₂O₃ layer thickness proportion of total TGO in LM-TBC is always greater than or equal to that in AS-TBC at the same isothermal oxidation time. Compared with the AS-TBC, the parabolic oxidation rate of LM-TBC is decreased by 18.42%. Therefore, YAG/YSZ LM-TBC presents better oxidation resistance and lower TGO growth rate than AS-TBC.