Abstract: In order to investigate air breakdown caused by short-pulse HPM (high-power microwave), the effective electric field, an important parameter for the calculation of ionization parameters, was obtained by solving the electron momentum equation for air. The effect of altitude on the air breakdown thresholds of narrowband and ultra-wideband HPM pulses and the relationship between escaping time or energy and pulse amplitude were given by substituting the effective ionization parameters into and solving the continuity equation of electron density. In addition, the energy transmission of an ultra-wideband HPM pulse under the condition of air breakdown was analyzed. The research results show that altitude, pulse width, spectrum distribution and amplitude have a significant impact on the air breakdown.