Abstract: In order to improve the accuracy of eddy current testing of steel bar with small cracks and reduce the nonlinear error due to select inappropriate testing parameters, 25^# steel bars with 0.1 mm crack depth were detected by using the eddy current detector and the external probe under different frequency and lift-off value based on experimental model, and then the phase information was obtained. The influences of working frequency and lift-off value on the phase signals were studied for the eddy current testing. According to the minimum principle for the average standard deviation of linear fit, the best linear section was achieved between the detection parameters of the eddy current testing signal and phase parameter, which raises the detection accuracy of the bar with small cracks, and has important guiding role for choosing the eddy current testing parameters of external bars.