The ultrafast pump-probe method is introduced into the optical vortex coronagraph technique， which can realize time-resolved background free imaging of a weak phase object. Specifically， the proposed technique is applied to the diagnostic of air plasma pumped by femtosecond laser pulses. Experimental results show that the formation process of air plasma can be clearly measured. The numerical simulation results are consistent with the experimental results. In addition， two decay processes of the plasma are identified and fitted by double exponential decay. The fast process with a short time scale of 65 ps is attributed to the recombination of electrons and positive ions， while the other relatively slow process with a long time scale of 810 ps is due to the attachment of free electrons to oxygen molecules.