In the vicinity of atmospheric breakdown threshold, microwave pulse breakdown discharge becomes a probabilistic problem, which is closely related to breakdown time. When atmospheric breakdown occurs, the transmission pulse width and peak power will be shortened. The development of high power microwave technology is severely restricted by the hazards. To study the breakdown problem in high power microwave atmospheric transmission, the probability distribution curves of breakdown time in different processes are compared, and the effects of ionization rate and seed electron generation rate on breakdown time are discussed. The breakdown process of free space microwave was experimentally studied using S-band high power microwave pulse source. The breakdown time of plasma was monitored by photomultiplier tube, and a series of experiments were carried out in the presence or absence of cesium 137 seed electron source. The results show that increasing the seed electron generation rate is a more effective method to increase the pulse breakdown probability. In the process of repetition frequency, if there is cumulative effect, the breakdown delay time probability distribution curve will move to the left and tend to be stable. And the gas after breakdown will be easy to breakdown again in a short time.