As a special bistatic configuration, the imaging model of spacemissile borne bistatic synthetic aperture radar (SMB-BiSAR) is firstly constructed in vector according to the geometry of satellite and missile the range history and the echo signal model being obtained. Then, the constraint of the missile flight path is analyzed based on the orthogonality of isorange and isoDoppler contours. Finally, the spatial resolution expression of SMB-BiSAR is derived by the gradient of range history and Doppler frequency. The simulations show that SMB-BiSAR has the ability to obtain the highresolution image of target area in the case of large squint angle and forwardlooking by the path planning of missile. In addition, the bistatic geometry has greatly effects in spatial resolution. The greater the bistatic angle between transmitter and receiver is, the worse the range resolution becomes. When both the transmitter and the receiver overlap, i.e. the range resolution, is the best. The larger the angle between moving direction of transmitter and receiver is, the worse the azimuth resolution is obtained. And the azimuth resolution is the best, when the motion direction is consistent.