Abstract: High-power high-frequency transformers are key components in power electronic transformers, and the global optimality and computational speed of their optimization design are key issues. This paper proposes a high-speed global optimization method based on improved thermal analysis and co-design with converters for high-power high-frequency transformers. For thermal analysis occupying a high proportion of time consumption in optimization, an improved thermal analysis method is proposed based on heat dissipation- conduction two-step simulation, which not only ensures the accuracy of thermal analysis, but also improves the computational speed. Simulation results validate the accuracy and the effectiveness of the proposed thermal analysis method. With the aim of improving the performance of the design scheme, during the optimization process, the converter circuit parameters and the transformer inductance parameters are considered together, and more optimization variables are introduced, so that global optimization model is formed. Besides, the improved thermal analysis method and the genetic algorithm are adopted to increase the computational speed and ensure the computability of the global optimization model. Integrating the above approaches, a systematic high-speed global optimization design process for high-power high-frequency transformers is formed. A specific case design and experimental results validate the effectiveness of the proposed optimization method.