Abstract: A general geometry model of airborne bistatic spotlight SAR is firstly established, and the echo expression in time-frequency domain is presented. Secondly, the difference frequency processing is performed by using the echo of the imaging scene center as the reference. The difference distance obtained is expanded into Taylor series. Then, the three-dimensional spatial resolution of bistatic spotlight SAR is formulated by using the first-order approximation under far-field assumption. Theoretical results are illustrated by numerical simulations. Finally, based on a general bistatic configuration with the transmitter and the receiver flying along the horizontal direction and the sloping direction, the paper analyzes the variation rules of the spatial resolution following by the geometric configuration and the motion parameters.