The azimuth resolution of Spotlight SAR is analyzed from the azimuth antenna pattern weighting, and the effect of the azimuth-varying Doppler on Spotlight SAR interferometry is studied. A coregistration algorithm based on adaptive subspace projection is proposed for Spotlight SAR interferometry. In this method, an adaptive factor is introduced to select the sample number of joint covariance matrix based on the coherence value. Moreover, the Doppler centroid drift in the azimuth direction is also considered in spotlight SAR images coregistration. The method is validated with the real data from the GERMAN radar satellite TerraSAR-X.