Abstract: With the application of large number of power electronic devices, traditional sinusoidal steady\|state or quasi\|steady state assumption in the analysis of fast transient problem can't meet the requirements of both speed and accuracy. Dynamic phasors model used to analyze this problem is more precise than by traditional steady\|state model, its simulation results are very close to that obtained by the time domain simulation, and its simulation is fast. In this paper, dynamic phasors method is used to build a mathematical model of the converter power electronic switching devices. The Fast Fourier (FFT) analysis shows that the time\|domain simulation results of inductor current and capacitor voltage is close to the results obtained by using of DC component, which simplify and improve the dynamic phasors model of Buck\|boost converter. The simulation results show that the model has features of nonlinear and time\|invariant, and can accurately reflect the transient dynamics of converter.