A new scheme of the automatic acquirement of target's region-of-interest (ROI) by utilizing SAR images on the study of earth observation is proposed based on a simple review on the progress and the analysis of existing problems. The new scheme consists of a scheme of target detection and that of target discrimination. The key techniques of the new scheme are also developed. The experiment results of real SAR scenes show that the presented scheme has the characteristics of strong robustness, extensive practicability, high automatic on low computation and easy realization. Therefore, the presented scheme has the extensive prospect for spatial earth observation.