Abstract: A high-power microwave easily causes the air breakdown, and the produced plasma has an obvious influence on the microwave propagation. In this paper, the transmission, reflection and absorption of 110 GHz high-power microwave in air breakdown plasma at atmospheric pressure is studied using a electron fluid model. Simulations show that, the filamentary plasma structure in 110 GHz microwave air breakdown similar to the experimental phenomenon is reproduced; the reflection and absorption of the microwave by the breakdown plasma change with time because the plasma is a time-varying media; with time increasing, the plasma absorption increases, till it reaches the saturation level that is much higher than the reflection by the plasma. As the incident electric field decreases, the reflection by the breakdown plasma becomes lower. The breakdown threshold of 110 GHz high-power microwave obtained by simulations agrees well with the experimental data.