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With the development of narrow-band high-power-microwave (HPM) technology, air breakdown has become a serious problem that concerns to the HPM applications. A new method is presented in this paper to determine the electric field of the microwave air breakdown near the atmosphere pressures. Two metal pins were used to strengthen the microwave electric field in a sealed waveguide, putting on the symmetry axis. Some parameters such as gas pressure, temperature, and humidity could be changed conveniently to achieve gas breakdown. Experiment was carried out at the S-band (2.86-GHz) microwave of 180-ns duration and 5-MW peak power, and typical phenomena of plasmas introduced by microwave are analyzed. Furthermore, a new definition of breakdown threshold has been proposed based on breakdown probability, and the electric field threshold is also analyzed.