Medium-voltage and high-capacity converters are vital for new energy generation, petroleum metallurgy, and electrical drives. Their long equipment service life and high cost make the reliability enhancement of them crucial. Among various failure factors in large-capacity converters, the failure of Insulated Gate Bipolar Transistor(IGBT) modules due to temperature-related issues is the most significant component. Therefore, real-time temperature monitoring of IGBT modules is one of the key means to improve the reliability of large-capacity converters. To reduce the influence of material temperature variations, aging, and power loss calculation errors on the junction temperature estimated by the thermal resistance model, the researchers are increasingly paying attention to the temperature observer, which imposes requirements on the observability of the model’s state space. A hybrid thermal resistance is proposed to solve the problem that existing thermal resistance models cannot simultaneously achieve high accuracy and strong observability for air cooling IGBT modules. The proposed model exhibits an error of less than 5% compared to finite element analysis. The lumped thermal resistance model for IGBT in this paper can be implemented in the controller to monitor the 3D temperature distribution in real-time, and can serves as the basis for a temperature observer.