Abstract: A set of equations for the non-linear propagation of high power microwave pulses in the atmosphere as well as the evolution of self-generated air plasma are deduced. A 1D code named Atmospheric Plasma Non-linear Evaluation in code (APNEIC) has been programmed in a local time domain reference frame moving with group velocity of the electromagnetic wave. The evolution of character parameters of the self-generated plasma such as the electron number density, the electron temperature, and the vibration temperature are studied. The simulation results show that during the breakdown startup stage the electron temperature is enhanced very rapidly to saturation values because of the rather low local seed free electrons, while the increase of electron numbers is relatively slow, but the electron temperature decreases soon from its saturation value with the avalanched increase of electrons.