Abstract: Power transformers are among the most critical equipment in power systems, and their operational status directly affects grid safety and stability. A systematic study on digital online monitoring technology for transformers. is presented Four technical approaches of fiber-optic temperature sensing are analyzed, The oil-gas separation methods and gas detection sensors used in dissolved gas analysis (DGA) online monitoring are also compared. Based on these analyses, a five-layer digital online monitoring system architecture comprising equipment, sensing, terminal, platform, and application layers is designed. The winding hot-spot temperature calculation model based on GB/T 1094.7—2008 and IEC 60076-7, as well as the dissolved gas analysis fault diagnosis method based on DL/T 722—2014, are introduced. The effectiveness of the system is validated through engineering applications. The research results have significant engineering application value for transformer condition monitoring and predictive maintenance.