Abstract: Surface nanocrystallization was carried out on Cr5Mo steel by using an ultrasonic shot peening (USSP) technique. The microstructure and microhardness on surface layer were studied, and the flow-accelerated-corrosion properties of the sample with and without USSP treatment in flow deionized water solution containing H2S were conducted, the morphology of corrosion product film was also observed. The results show that nanaocrystalline layer with a certain depth and mean grain size of 20 nm was obtained on Cr5Mo steel surface after USSP treatment. The microhardness of the sample surface was remarkably enhanced by surface nanocrystallization, the long the treatment time, the high the microhardness, and the large the deformation layer thickness. Compared with the sample without USSP treatment, flow accelerated corrosion (FAC) resistance performance of the samples with USSP treatment was promoted and the corrosion product film was compact. Tightness and integrality of the film decreased with the increase of USSP treatment time, as thus, the corrosion resistance declined gradually.