In this paper, an improved baseline estimation approach based on the Fourier analysis of the interferometric phases is presented. Four improvements are made: a precise method to calculate the instant phase frequency is given; the rule of distributing the interferogram samples is given to improve the ill condition of the equations of least square method; a quantitative criterion of choosing the interfrogram samples is presented which can exclude the influences of interferograms corresponding to areas of low correlation or strong topography variations; an iterative scheme based on the differential interferometric phases is given. The improved approach is used in the processing of the airborne dual-antenna and repeat-pass InSAR data. The results validate that the improved approach is more accurate and robust than the original one. This improved approach is suit for the baseline estimation of the airborne repeat-pass InSAR data.