The invention discloses a method for testing the CPU utilization of an intelligent-substation-monitoring background mainframe and aims to detect event distinguishing reliability the basis of the CPU utilization rate. The method includes the steps of firstly, connecting a computer with the standby network interface of a station-level central switch, and using the computer to acquire a station-level message; secondly, copying an acquired data packet to a network tester, and using the network tester to analyze data frames in the data packet to obtain data frame information; thirdly, recording the CPU utilization rate information of the monitoring background mainframe when testing is not performed; fourthly, screening and analyzing the data frame information, selecting the optimal data frame, and using the network tester to amplify the data frame until the port of the switch is fully loaded; fifthly, performing analysis and observation to obtain the CPU utilization rate information of the monitoring background mainframe. The method has the advantages that high testing efficiency and safety are achieved, and the CPU utilization of the substation-monitoring background mainframe can be tested.