The invention relates to a state diagnosis method for a small-current ground fault line selection device, which comprises the following steps of: firstly, sequentially doing an instantaneous ground fault experiment for a plurality of lines connected on a bus by utilizing the small-current ground fault line selection device to be diagnosed, wherein if the lines of which zero-sequence current phases are lagged behind a zero-sequence voltage phase of the bus by 90 degrees are more than the lines of which the zero-sequence current phases are ahead of the zero-sequence voltage phase by 90 degrees in the ground fault experiment, the PT (Potential Transformer) connecting mode of the bus is correct, or the PT connecting mode of the bus is wrong; and then, on the premise of determining that the PT connecting mode of the bus is correct, according to a phase relation of a zero-sequence current of each line and a zero-sequence voltage of the bus of a substation, judging whether the CT connecting mode of each line is correct. According to the invention, the connecting fault of the small-current ground fault line selection device can be rapidly and accurately diagnosed and the debugging period of the line selection device is shortened, so that the maintenance cost of the line selection device is reduced.