Abstract: In the power system, the stable operation of primary equipment in substations is crucial for ensuring power supply reliability. With the advancement of smart grid construction, online monitoring technology has become a key means of equipment condition assessment. This paper takes a main transformer in a 220 kV substation as the research object. Based on its online monitoring data such as oil chromatography analysis and partial discharge, an in-depth data diagnosis is conducted. Targeted operation and maintenance strategies are then formulated according to the diagnostic results. Based on the analysis and application of actual data, the effectiveness of online monitoring data in equipment condition assessment and fault early warning is verified, providing a practical reference for the reliable operation and maintenance of primary equipment in substations.