The invention relates to a measuring tool for measuring the main circuit resistance of a switch cabinet. The measuring tool comprises an insulating rod. One end of the insulating rod is fixedly provided with a conductive plate for conductive contacting with the copper bar of a main circuit to be measured. The conductive plate is provided with a current signal output portion for connecting the current signal line of a resistor. The conductive plate is equipped with a probe in an electrically insulating manner for conductive contacting with the copper bar of the main circuit to be measured. Theprobe is provided with a voltage signal output portion for connecting the voltage signal line of the resistor. The corresponding voltage signal and the current signal are collected by conductive contacting the conductive plate and the probe with the corresponding copper bar, and the conductive plate and the probe are sent into a cabinet body by the insulating rod, so that the measurement of the main circuit resistance can be easily and conveniently performed outside the cabinet body.