Abstract: Combing Maxwell equations and electron liquid equations, air ionization and breakdown caused by high power microwave(HPM) are analyzed using FDTD method. Comparing with the traditional approximate analysis, this method uses instantaneous electric field instead of equivalent electric field, and updates ionization frequency and collision frequency in each time step, so it can eliminate the error of analysis methods, which ignore the attenuation of electric amplitude during atmosphere breakdown. The change tendency of computed air breakdown threshold with altitude matches well with those in references, while the threshold values are a little larger. Finally, the impact of HPM amplitude, pulse width and altitude on air breakdown threshold are analyzed using this method.