Abstract: This paper simulates the physical processes of propagation of intense and short electromagnetic pulse in a rectangular waveguide applying electron fluid equations and Maxwell’s equations.It presents some characteristic parameters germane to air breakdown for different pressures, which include breakdown-plasma peak electron densities, peak electron fluid energies, breakdown positions in the waveguide,sneak-through times and fractions of electromagnetic energies transmitted.Onc can see“tail-erosion”phenomenon of the high-power microwave(HPM)pulse in air for uniform initial electron density; as well as“splitting”process for Gaussian ini-tial electron density.This paper also analyzes the physical mechanisms of various re-sults.