The process of high power microwave atmospheric breakdown is simulated with the combination of electron fluid equation and Maxwell equation. Using the characteristic parameters of plasma, the Finite Difference Time Domain(FDTD) method is adopted to analyze the atmospheric breakdown of the single pulse High Power Microwave(HPM) under different pressures and electric field intensities, then the time of the atmospheric breakdown is obtained under different pressures and electric fields when the microwave frequency is 6.4 GHz. Theoretical analysis and simulative results show that the change of the pressure and the electric field has a significant effect on the atmospheric breakdown time, because the electron concentration in the atmosphere is dominated by the electric field intensity and pressure. The atmospheric breakdown time is at 10 ns order of magnitude when the field strength is at the order of kV/cm. When the electric field strength is constant, it can be found that the atmospheric breakdown time will be reduced with the increase of pressure. Then the atmospheric breakdown time will increase again when the pressure drops to a certain value. Through the contrast analysis of different field strengths, it also can be found that the higher the electric field intensity, the shorter the atmospheric breakdown time.