Abstract: The aging and deterioration of interturn insulation in oil-immersed power transformers cause interturn insulation resistance to decrease, which results in an increase in current and heat generation on the faulty coils. Therefore, it is important to study the electrothermal characteristics of the transformer in the early stage of interturn faults. By analyzing the effect of interturn fault resistance on the transformer equivalent circuit and winding losses, a transformer interturn fault model based on coupling the electromagnetic and thermal-fluid fields is proposed in this paper. Based on the idea of digital twin (DT), a high-fidelity simulation model of the transformer is established to achieve the accurate simulation of winding currents and temperatures of different components by transferring the transformer entity's structure, size, and material parameters. The high-fidelity simulation model is used to study the electrothermal characteristics of the transformer with a 1% interturn fault in the high-voltage winding. The results show that the current amplitude of the low-voltage winding only increases by 2%, while the top oil temperature rises by 9.4℃ (25.7%), and the overall temperatures of the tank wall rise by 10℃. In the early stage of interturn fault, the changes in top oil and tank wall temperatures are more significant than the winding currents, so the electrothermal characteristic parameters such as top oil temperature, tank wall temperature and winding currents can be adopted to identify the early fault.