The work aims to determine causes of soft spot on parts after high frequency quenching and obtain qualified structure by adjusting process parameters. By reading chemical composition of spectrometry test workpiece directly, macrostructure and microstructure of samples were observed by stereo microscope, optical microscope and scanning electron microscope to analyze quenching layer depth and microstructure structure of workpiece. The composition difference in quenching structure of the samples was tested by energy spectrometer. The production process was sorted out to judge the possible causes of soft spot on quenching parts. Tested by the energy spectrometer configured by SEM, content of martensite in the black area of unqualified samples was high while that in the white area was low; for qualified samples, there was little difference in carbon content of martensite in black and white area. High carbon and low carbon martensite form in microstructure of the quenched parts and soft spot was present since high frequency quenched parts are not subject to prior thermal refining, carbon content distribution is not uniform, high frequency quenching time is short and carbon atoms are not fully diffused.