Aiming at the low intelligence level of main transformers in nuclear power plants, the fact that the operation and maintenance still mainly rely on manual inspection and regular maintenance, and the lack of real-time prediction and evaluation means of operating state, the digital twin of the main transformer based on fast calculation of coupling field has been built. Firstly, a refined digital model of the main transformer was established and the multi-physical field real-time simulation analysis was carried out to obtain the electromagnetic field and loss distribution cloud maps of its key components. Then, in order to solve the problems that the simulation of the flow field and temperature field of the main transformer took a long time and the real-time monitoring of operating states could not be realized, a fast calculation method of coupling field was proposed. Finally, the digital twin of the main transformer was constructed by combining the proposed method with the operating state data collected by various sensors, achieving multi-dimensional, full process and panoramic state monitoring and display for the physical main transformer. The results show that the construction of digital twins can realize the real-time monitoring and predictive evaluation of the operating state of main transformers under different working conditions, which provides convenience for the operation and maintenance inspection of the main transformer and contributes to the safe and reliable operation of nuclear power plants.