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The high-temperature oxidation resistance of AISI 321 stainless steel for solar thermal power generation heat exchanger highly determines its service life. Therefore, in this work, aluminizing treatment and aluminizing with subsequent laser shock peening (LSP) were employed to improve the high-temperature oxidation resistance of AISI 321 stainless steel at 620°C. The results showed that these two treatments decreased the oxidation rate as compared to the base AISI 321 steel. Concretely, the optimal oxidation resistance was observed in the aluminized steel before an oxidation testing time of 144 h due to the increased the entropy of the LSP-treated specimen. After 144 h, however, the LSP-treated sample showed the best oxidation resistance because of the formation of protective α-Al2O3. For the LSP-treated samples, the large amount of sub-grain boundaries formed on aluminized layer could act as the fast short-circuit path for the outward diffusion of Al element, facilitating the rapid nucleation of α- Al2O3. Meanwhile, the aluminized layer is able to isolate the contact between oxidation environment and matrix, thereby decreasing the oxidation rate. Further, the oxidation parabolic constant D(t) of LSP-treated steel was calculated to be minimum (6.45787×10–14), which is respectively 69.18% and 36.36% of aluminized steel and 321 steel during the whole oxidation process. Consequently, the combination of aluminizing and LSP can better improve the high-temperature oxidation resistance of 321 stainless steel.