Pipeline steels utilized in deep seawater are usually protected cathodically. However, inappropriate operations of cathodic protection systems cause hydrogen embrittlement failures to these high strength steels in seawater which result from the application of excessive negative potentials, leading to massive generation of hydrogen at the protected pipelines' surface. With high strength steels increasingly widely used in the deep sea environment, the basic research to the cathodic protection and susceptibility to hydrogen embrittlement of high strength steels under such a circumstance is still unfortunately relatively lack and urgently needed to supplement. Electrochemical measurement, hydrogen permeation current detection, slow strain rate tensile test (SSRT) and fracture morphology analysis, therefore, were employed to investigate effect of cathodic polarization level on the susceptibility of API X80 pipeline steels to hydrogen embrittlement in simulated deep seawater in the present work. The results showed that the applied cathodic polarization potentials significantly affected hydrogen permeation and hydrogen--induced cracking behavior of X80 steels immersed in deep seawater. A linear relationship was found between the hydrogen permeation current densities and cathodic polarization potentials applied according to the findings of the potential dynamic polarization and hydrogen permeation current measurements. SSRT tests suggested that X80 pipeline steels immersed in simulated deep seawater didn't show susceptibility to hydrogen embrittlement at open circuit potential, and thus the optimum cathodic protection potential range was supposed to be above －900 mV (vs saturated calomel electrode). Under such a cathodic polarization potential, the hydrogen permeation current densities of X80 pipeline steel specimens were less than 0.1157 μA/cm² andtheir mechanical properties didn't decrease remarkably. Once the cathodic polarization potentials lower than －900 mV, however, hydrogen permeation current densities and calculated hydrogen embrittlement coefficients ψ of X80 pipeline steels increased significantly, exhibiting higher susceptibility to hydrogen embrittlement in simulated deep seawater. Furthermore, macro－and micro－morphologies of fracture surface of X80 pipeline steels after SSRT test indicated that the fracture morphology transformed from a dimpled pattern with ductile fracture to a quasi—cleavage pattern when cathodically polarized to lower than －900 mV, i.e. showing obvious brittle failure characteristics.