Abstract: Firstly, the model about space geometry layout of bi-static SAR system is given, its range history and range change rate history are analyzed as well. Comparison of the result to the mono-static SAR demonstrates its specialty and complexity. Lastly, the point target signal of bi-static SAR system is analyzed, and its phase variation history and its character are derived, which provide the theory reference for the signal processing research of bi-static SAR system.