The hard spot detection is one of the key contents during quality evaluation of high-end steel plate products. In this study, a domestically developed instrument based on the principle of incremental eddy current method is presented for detecting hard spots in steel plates. The function and performance of the developed instrument are experimentally examined. First, a sensor of incremental eddy current and its detection circuit are developed by using AC-bridge, phase rotation and demodulation techniques. As a result, a specialized instrumentation of incremental eddy current is integrated. Secondly, several groups of experimental testing are performed on steel plate of Cr12MoV and pipeline steel of X70 with the developed instrument. The obtained results show that the spatial resolution of the sensor can reach around 10 mm. The peak-to-peak value (Up ) of the demodulated waveform of the incremental eddy current signal is used as feature parameter, which demonstrates approximately linear decreasing trend as the increase of the surface hardness. The value of Up is decreased by at least about 75% in the hard spot region in which the hardness is about 34. 8% higher than the baseline. Magnetic imaging of pipeline steel of X70 is realized by using the parameter of Up and the hard spots with a width around 10 mm is accurately identified from the imaging results. The above research results provide support in method and instrument for on-line, nondestructive testing of hard spots in high-end steel plates.