The invention relates to a method for testing cable insulation. The method includes the following steps that a tested cable is connected with a relay secondary gating module amphenol connector in a testing main control computer; the testing main control computer is communicated with an upper computer through 485 bus; the upper computer sets testing parameters and detection modes of the main control computer according to the measured cable demands, and the detection mode comprises circuit testing, manual designated testing and fault tracking monitoring; a tested cable range is selected from a management software interface of the upper computer; the main control computer tests an insulation resistance of the tested cable, a detection excitation voltage, a detection excitation current and a detection channel status according to the testing parameter of the upper computer and the detection modes, detection data is reported to the upper computer for display, when the detection data exceedsthe limit to alarm, the status is displayed through a color. The method for testing cable insulation has the advantages of guaranteeing real-time online detection of the tested cable under the workingstatus, ensuring the safety of a tested object and improving the reliability of testing.