A nanocrystalline layer was prepared on the surface of 34CrMo4 steel by time controlling shot peening (SP, i.e., 1, 5, 10, and 20 minutes). Field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD) analysis, and transmission electron microscope (TEM) were applied to analyze the surface, cross-sections, and grain size of the specimens before and after SP. The electrochemical corrosion behavior was used to simulate a liquid under the oil and gas wells environment. It was characterized by the potentiodynamic polarization test and electrochemical impedance spectroscopy (EIS). The analysis results show that the surfaces of the SP samples were very rough and had numerous cracks. A passive film on SP surface was formed by nanocrystalline grains. However, the passive film formed in the initial stage was not dense or uniform, and cracks occurred in the passive film during peening, resulting in a decrease in corrosion resistance.