Online temperature monitoring of IGBTs is a crucial means to enhance the reliability of high-power converters. In the existing thermal model methods, the junction temperature is derived through the device power loss, which is difficult to obtain accurately in real time. This paper proposes a power loss observer to estimate the real-time power loss accurately. Unlike conventional methods, the proposed method only needs measure the heatsink temperature. Moreover, the proposed technique is robust to disturbances such as wind speed fluctuations, solder aging, and temperature dependence of thermal parameters, which helps to improve the temperature estimation accuracy throughout the full life cycle of IGBT modules. This paper analyzes the proposed method's mathematical principle, algorithm, and implementation steps, while the loss observer is validated by simulation and experiment.