Abstract: The cathodic protection tests for internal surface protection of the intake steel pipes with different internal diameters by impressed current and sacrificial anode methods are carried out. Test results show that, for the small dimension pipe, the distribution of the potential at internal surface is non uniform, that the protection scope is small, and that the polarization potential at internal surface distributes more non uniformly in flowing fresh water than in resting water, while the potential is well distributed with a large protection scope for the big dimension pipe.