Abstract: This paper analyzes in detail a start-up loss event of No.1 main transformer in a 220 kV substation, explains in detail the process of main transformer start-up failure and the troubleshooting method, and comprehensively analyzes the possible causes of failure in combination with electrical principles. Circumstances determine the ultimate cause of failure. Finally, this paper analyzes the potential causes of the failure, and gives the arrangement of the next work.