Among a large number of time series of SAR images, selection of all valid coherent targets can improve space density and guarantee reliability of SAR interferometry for deformation monitoring. Continuous observation of ground based SAR can get local area deformation day and night in all weather conditions, and has the characteristics of large amount of data, zero baseline interferometry, strong coherence and different spatial resolution. In this paper, based on the imaging characteristics, a double threshold method of extracting coherent targets from a large number time of series ground based of SAR images is proposed. The method is tested on Geheyan Water Conservancy project area with 1 330 ground based SAR images. Experiment results prove that the method can effectively extract reliable coherent pixels. Moreover, when the number of ground based SAR images is more than 600, less than 0.3 of average correlation coefficient threshold almost has no effect on coherent targets number, while the amplitude dispersion index threshold is the main factor of effecting the number of coherent targets in ground based SAR images.