The spacial spectrum of the interferogram of plasma induced in air by high power infrared laser radiation is analysed. And the phase information' successfully is extracted from the interferogram by using high-order ChebyShev digital recursive filters and the phase unwrapping algorithm with ttrieshold auto-adjustment. The designing considerations of high-order digital recursive filters are described and experiment results are given in this peper. The results show that this approach can be used for extracting phase information from high-speed or instant optical interferograms.