Abstract: The high-frequency transformer is a crucial device in a dual active bridge (DAB) converter. DC bias may occur during operations. Constructing a high-frequency transformer model with the capability to accurately represent the nonlinearity of the magnetic core is the basis to accurately analyze the electromagnetic transient characteristics involving core saturation, such as direct current (DC) bias. The key lies in accurately measuring the magnetization curve of the high-frequency transformer. In this paper, the DAB generates an equal amplitude variable frequency square wave voltage by switching frequency modulation, and the intermediate high-frequency transformer is excited by variable flux linkage. Magnetization curves and hysteresis loop clusters are obtained through calculations based on the voltage and current data measured at the port. This method does not need to add any AC excitation power supply, and can realize the measurement of magnetization curve of high frequency transformer by switching modulation when DAB converter is put into operation, which can solve the technological problems of inefficient and high-cost measurements for the magnetization curves of high-frequency transformers. A DAB platform is built to measure the magnetization curve of a test high-frequency transformer and its electromagnetic transient model is built. The rated frequency overvoltage and DC bias experiments and simulaitons are carried out. The model simulation errors are all within 7%, and the maximum mean square error(MSE) is only 0.002 7. The results show that a high-frequency transformer electromagnetic transient model that considers excitation nonlinearity can greatly improve the simulation accuracy of core saturation conditions, and it is of great significance to the electromagnetic transient characteristics analysis and control of DAB.