Abstract: In recent experiment about air breakdown induced by high power microwave, when the incident microwave value is set near the breakdown threshold, it can be found that although all the other external conditions are the same, it is not certain that the air breakdown will occur. To solve this problem, the mechanism of air breakdown was studied and we divided air breakdown into two stages, one stage was the appearance of the first free electron in the breakdown region, the other was the avalanche induced by microwave electric field accelerate electron. A novel model combining average electron generation rate model and avalanche breakdown model was proposed to simulate the probability distribution of high power microwave breakdown in air. The air breakdown probability distribution was simulated under conditions similar to the experiments, and the simulated results agreed well with the experiment data.