To address the issue of low accuracy in calculating transformer core losses under high magnetic flux density conditions, it is necessary to establish a more precise mathematical model for core loss. This paper modifies the calculation coefficients of the Bertotti model to develop a new mathematical model. By designing a silicon steel sheet loss measurement experiment, core losses under different magnetic field conditions were obtained, enabling the establishment of an improved mathematical model and its comparison with classical models. The study reveals that traditional calculation methods exhibit significant errors under high magnetic flux density, whereas the new mathematical model substantially reduces such errors. The research demonstrates that the improved Bertotti model effectively enhances the calculation accuracy of core losses under high magnetic flux density, providing a theoretical foundation for the optimal design and performance improvement of transformer cores.