The invention provides a signal acquisition circuit, a signal acquisition device and a signal acquisition system applicable to magnetic steel online fault diagnosis. The signal acquisition circuit comprises a signal connecting module, a magnetic steel noise signal acquisition module and a magnetic steel internal resistance signal acquisition module, wherein input of the signal connecting module is connected with two ends of a magnetic steel coil, and output of the signal connecting module is connected with a channel switching module and connected with a corresponding signal acquisition module according to diagnosis requirement; the magnetic steel noise signal acquisition module is connected with the output of the signal connecting module through the channel switching module when magnetic steel noise detection is required to be performed, so as to acquire a magnetic steel noise signal, amplify the magnetic steel noise signal and then output the amplified magnetic steel noise signal; and the magnetic steel internal resistance signal acquisition module is connected with the output of the signal connecting module through the channel switching module when magnetic steel internal resistance detection is required to be performed, so as to acquire a magnetic steel internal resistance signal, perform filtration and impedance conversion on the magnetic steel internal resistance signal and then output the magnetic steel internal resistance signal. The signal acquisition circuit, the signal acquisition device and the signal acquisition system are capable of avoiding a situation that management data of other tracks are influenced when diagnosis is performed through more shafts or less shafts in the prior art.