Abstract: Cathodic polarization behavior of an X70 pipeline steel crevice in solution containing Cl^- and SO₄^2- was studied. Experimental results show that at a protection potential of -1000 mV vs CSE, with increasing crevice size the potential of the crevice tip is close to the protection potential; at the same crevice size, with the increase of protection potential the status of protection is easily reached in the crevice. Under unprotection condition, crevice corrosion can happen and the pH value of solution reduces in the crevice; under protection condition, the pH of solution can rise and in the crevice the solution alkalized.