The intensity thresholds of air ionization induced by 800nm ultrashort laser pulses are investigated for laser pulse durations from 50fs to 22ps. The intensity threshold was determined by measuring the fluorescence energy of the stimulated air ionization plasma. It is shown that as the pulse duration increases from 50fs to 22ps, the threshold intensity of air ionization decreases from 8.7×1014 to 2.7×1013W/cm2 and undergoes the transition from rapid decrease to slow decrease. For pulse duration shorter than 1ps, the theoretical calculation was done based on the multiphoton ionization and for pulse duration longer than 1ps, the calculation was done based on the impact ionization initiated by the multiphoton ionization. For both above cases the calculated results are in good agreement with the experiment.