This paper proposes an alternating current electric flux leakage (AC-EFL) testing method, which was verified by a series experiments. The inspection ideal and its feasibility research were detailedly presented on the basis of electric field (E-Field) theory. The inspection principle, which is to evaluate surface defect in AC-carrying metallic components using a coplanar capacitive sensor, was briefly described. Physical experiments with a prototype capacitive probe were also carried out to observe and analyze the detection performance of AC-EFL. Effects of crack size of the conducting specimen, through holes size, flat-bottom holes depth, insulation thickness between the probe and the conducting specimen, and exciting signal frequencies were studied. The proof-of-concept results indicated that the AC-EFL testing method could be used to detect surface defect for metallic specimen, and of complete feasibility and the potential of great practical value as a supplement of conventional NDT methods.