The electrochemical behavior and pitting corrosion in a Cl^- containing solution (0.05 mol/L H₂SO₄+0.05 mol/L NaCl) of the ultrafine-grained 304L stainless steel (304L SS) with average grain size of (130±30) nm prepared by equal channel angular pressing (ECAP) technique were examined using potentiodynamic polarization curves, cycle polarization curves, electrochemical impedance spectroscopy (EIS), Mott-Schottky (M-S) curve measurements together with SEM observation of surface morphology. As compared to the coarse-grained counterpart, the ultrafine-grained sample exhibited a higher corrosion current density i_corr of 81.74 Acm² and a lower corrosion potential E_corr (vs SCE) of -466 mV, and having a higher passivation current density i_p of 32.38 mAcm² and a narrower passive region (-315~450 mV) together with a breakdown potential E_b decrease of 100 mV and a protection potential E_bp decrease of 190 mV. On one hand, the grain refinement induced by severe plastic deformation deteriorates the compactness of the passive films and is helpful for the Cl^- absorption, resulting in a 1.6 times increase of the carrier density and one order of magnitude increase of the diffusion coefficient in the passive films. On the other hand, the significant increase of grain boundaries provides more possibility for Cl^- diffusion along grain boundaries, and thus promotes the pitting nucleation and growth.