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This paper sets up a simulating model of single-phase transformer under DC bias in MATLAB. Using MATLAB software, the magnetization characteristic curves and exciting currents under no-load condition are simulated and analyzed. The result shows that the magnetization characteristic curves do not shift up, but they are no longer symmetrical under DC bias. They get saturate at one side of X axis. The hysteresis loops become narrow and tend to single-valued curve when DC increases. The corresponding peak values of exciting current get sharp, and the waveform of exciting current is no longer symmetrical too. The harmonics content become more and more abundant. The 1st and 3rd order harmonics increase steadily with increases of DC. Especially the 2nd order harmonic increases rapidly with increases of DC. A few of 4th order harmonic arises.