The invention discloses a level testing system, comprising tested equipment, a linear impedance stabilization network, a signal coupler and an electro-magnetic Interference (EMI) test receiver, wherein the linear impedance stabilization network is connected with the tested equipment; the linear impedance stabilization network provides power frequency voltage with linear stable impedance to the tested equipment; the first input end of the signal coupler is connected with a center line between the linear impedance stabilization network and the tested equipment, and the second input end is connected with a phase line between the linear impedance stabilization network and the tested equipment; the EMI test receiver is connected with the output end of the signal coupler; the signal coupler couples a differential mode interference level of the tested equipment to the EMI test receiver; the EMI test receiver tests the differential mode interference level, and outputs the test result. By adopting the level testing system, test of differential mode interference to the tested equipment is achieved.