The electromagnetic interference problem of Power electronic devices runtime is more and more serious in the complicated electromagnetic environment, thus putting forward the more strict requirements on the system itself. The quantitative prediction and effective suppression of electromagnetic immunity become a technical problem. This article is written for the electromagnetic immunity problem of power electronic devices, making a test and the switching power supply (model for NET-50B) does conduction immunity test in both no-load and load resistance cases. The interferential voltage (140 dBuV) is injected in switch power supply through the coupling/decoupling network, and testing the output voltage stability under different frequency, also monitoring the radiation in the process of test by using the spectrum analyzer. The electromagnetic immunity of this switch power supply is obtained through test, verifying the validity of the test method, and it is a great significance for predicting and analyzing the electromagnetic immunity of power electronic equipment.