Main transformer is the essential part of the power system in a nuclear power plant, which is responsible for transmission of high-voltage electricity and distribution of low-voltage electricity, so as to reduce power loss. This paper generally describes the situation of main transformer as well as the relevant operation management. By analyzing a case of main transformer shutdown, this paper has revealed the influence of main transformer failure event on nuclear safety system, studied the problem of conductor oxidation caused by insufficient design margin of low-voltage compartment bushing soft connection circuit contact surface, and put forward some targeted improvement suggestions. In this paper, the common problems extracted from main transformer failures are analyzed from the aspects of design, manufacturing, and maintenance management, including the design defects in lead terminal, iron core fastening, and magnetic flux leakage, the manufacturing defects in foreign matter control, sealing and assembly, casing quality and other areas, as well as maintenance management defects like imperfect procedures or poor control of foreign matters, etc. Apart from discussions on the main factors affecting main transformer life and how to carry out failure risk management, this paper has proposed suggestions on strengthening design review, manufacturing supervision control, installation and commissioning specifications and operation and maintenance in closed-loop management, and has introduced the application of optical fiber temperature measurement technology in the real-time monitoring of main transformer internal hot spot temperature, which provides data support for main transformer load forecasting, life assessment and condition assessment.