An instrument interface method and device. Two capacitors, one capacitor has one end as input of the device, connected to live line of power output of a LISN, and has other end as output of the device, connected to one test port of an oscilloscope; the other capacitor has one end as input of the device, connected to neutral line of the power output of the LISN, and has other end as output of the device, connected to another test port of the oscilloscope; without changing the LISN design, existing LISN products can be used for conducted emission test with oscilloscope-based time-domain EMI measurement instruments by means of the method and device. Said two capacitors have a capacity of