A geometry model of bistatic SAR (BiSAR) is provided, which can quantitatively describe the different degree between transmitter and receiver trajectories. The proposed geometry model is derived on the assumption that the earth is local flat and transmitter and receiver antennas are at the side-looking mode. From the point of view of system analysis, the comprehensive properties of airborne, including spatial resolution, baseline range and bistatic angle, are envisaged based on the proposed geometry model. Some characteristics of BiSAR system are exhibited via overall simulation experiments and these appreciated results are helpful to gain further insight into design issues.