In response to the possible breakdown phenomenon of high-power microwave in atmospheric transmission, our study focuses on the first breakdown delay pulse number in pulse sequences. It is found that it is closely related to seed electrons, pulse breakdown probability, and microwave field strength. Microwave field strength can indirectly affect the pulse breakdown probability and delay pulse number through seed electrons. A method is proposed to estimate the critical field strength of microwave breakdown using the number of delayed pulses, and the microwave critical field strength is defined as the breakdown threshold when the probability of pulse breakdown is greater than a certain value. In this paper, the estimation formula of pulse impulse breakdown probability is derived, and the performance of the estimator is analyzed. Then, the experimental verification is carried out using the S band microwave atmospheric breakdown simulation device. The experimental results show that, within a certain range, the number of pulse delays for repetitive frequency microwave pulse breakdown is only inversely proportional to the seed electron generation rate and pulse width, and can be used to estimate the probability of pulse breakdown, thereby giving the critical field strength for breakdown.