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The threshold of laser-induced breakdown of air is determined in experiment and a new theoretical model is established to calculate it.The model divides the breakdown process into two stages: the first one is multiphoton ionization process(MI)which provides enough “seed electrons” to initiate the next stage and the second one is predominated by cascade ionization(CI)which continues to produce free electrons geometrically until the critical free-electron density for breakdown is reached.We solve the differential equation set under our laboratory conditions.It is found that the time node dividing the two stages is at about 7.6 ns,and the calculated threshold is in good agreement with the experiment.