With the development of power system automation, the requirements for power supply reliability are getting higher and higher, so fault detection of electrical equipment has become the focus of research in power systems. With the increasing importance of condition detection, mature online monitoring and fault detection technology for electrical equipment becomes more and more important. It is not only the basis for condition maintenance, but also can help us achieve smooth operation of electrical equipment and extend the power equipment's life. This paper analyzes the common faults in transformer operation, and then studies and analyzes the theory and practice of online detection technology. Finally, the development trend of detection technology is pointed out.