Abstract: After comparing the electrochemical impedance of titanium alloy TC4 with different tension strains, it is considered that the corrosion rate of TC4 with high strain would be more easily affected by the frequency of corrosive current than that without strain. According to traditional stress corrosion mechanism, the stress corrosion crack was in a state of open and close alternately, and from this it made AC current. So the crack propagation would be affected by the frequency of the AC current, and when the frequency was 50 Hz, the corrosion rate was estimated to increase at least 10 times. It is concluded that in stress corrosion process the rate of crack initiation and propagation would be determined by the frequency of AC current on metal/solution interface.