A cascaded multilevel converter self-test system comprising a cascaded multilevel converter and self-test device, wherein the cascaded multilevel converter comprises at least two converting circuits which are cascaded, and the self-test device comprises at least one current detecting circuit, a voltage acquiring module and a calculating module, and wherein: the at least two converting circuits each have a first output terminal and a second output terminal and are electrically connected in sequence with the first output terminal and the second output terminal, and each of the at least two converting circuits is provided with and electrically connected to an external direct current source in a one-to-one manner, the direct current source is configured to supply a direct current to the converting circuit corresponding to the direct current source; a first terminal of the at least one current detecting circuit is electrically connected to the first output terminal of the first one of the at least two converting circuits and/or the second output terminal of the last one of the at least two converting circuits, a second terminal of the at least one current detecting circuit is grounded; the at least one current detecting circuit is configured to detect a first detected current in a case that the cascaded multilevel converter is in a first conducting state and a second detected current in a case that the cascaded multilevel converter is in a second conducting state; the voltage acquiring module is configured to acquire first bus voltages of the at least two converting circuits in a case that the cascaded multilevel converter is in the first conducting state and second bus voltages of the at least two converting circuits in a case that the cascaded multilevel converter is in the second conducting state; and the calculating module is configured to calculate an insulation resistance value of the cascaded multilevel converter based on the first detected current, the second detected current, the first bus voltages of the at least two converting circuits and the second bus voltages of the at least two converting circuits. the at least two converting circuits each have a first output terminal and a second output terminal and are electrically connected in sequence with the first output terminal and the second output terminal, and each of the at least two converting circuits is provided with and electrically connected to an external direct current source in a one-to-one manner, the direct current source is configured to supply a direct current to the converting circuit corresponding to the direct current source; a first terminal of the at least one current detecting circuit is electrically connected to the first output terminal of the first one of the at least two converting circuits and/or the second output terminal of the last one of the at least two converting circuits, a second terminal of the at least one current detecting circuit is grounded; the at least one current detecting circuit is configured to detect a first detected current in a case that the cascaded multilevel converter is in a first conducting state and a second detected current in a case that the cascaded multilevel converter is in a second conducting state; the voltage acquiring module is configured to acquire first bus voltages of the at least two converting circuits in a case that the cascaded multilevel converter is in the first conducting state and second bus voltages of the at least two converting circuits in a case that the cascaded multilevel converter is in the second conducting state; and the calculating module is configured to calculate an insulation resistance value of the cascaded multilevel converter based on the first detected current, the second detected current, the first bus voltages of the at least two converting circuits and the second bus voltages of the at least two converting circuits.