Networking brings advantages of data fully sharing in smart substation, but at the same time, network stress also brings risks of operation safety and reliability. Therefore, to find out the reasons of network stress generated, to research the manifestations of network stress, and to test the performance of protection devices under network stress, are particularly important. By researching the causes and manifestations of network stress, this paper gives a network stress test method of relay protection in smart substation and gives the technical requirements. By simulating the packets under different network stress situation, this test method can test the performance of protection devices during the broadcasting packets network stress and unsubscription GOOSE/SV packets network stress. Also it can test the performance during the subscription packets network stress completely and systemic, which ensures the protection devices operating safe and stable in smart substation network environment.