In this paper, a new approach for the understanding of the SAR image of the cylinder oil tank is presented. In the approach, the Physical Optics (PO) and the theory of Incremental Length Diffraction Coefficients (ILDC) are employed to set up the scattering model of the oil tank according to the circle edges and cylinder geometry futures, and the position of the main scattering centers are deduced. The SAR image of the tank is simulated through the scattering model and the imaging model based on Mapping and Projection Algorithm (MPA). The real SAR data and the simulation test the validity of the approach. Furthermore, the scattering centers are detected with 45 groups of the measured data. According the scattering model and the imaging model in the approach proposed, the main scattering characteristics of the cylindrical oil tank are interpreted and summarized. It is hoped that the work in this paper could provide some useful guidelines for the dentification of the oil tank in the SAR images.