Provided is a method and device for obtaining internal side and external side insulation resistances of a relay. The method includes steps of controlling the insulation resistance obtaining circuit to output a low-frequency AC signal; when both the main relay and the pre-charge relay are switched off, obtaining an internal side insulation resistance of the main relay according to the low-frequency AC signal; if the internal side insulation resistance of the main relay is normal, controlling the pre-charge relay to be switched on; and when the main relay is switched off and the pre-charge relay is switched on, obtaining an external side insulation resistance value of the main relay according to the low-frequency AC signal.