Spaceborne/airborne bistatic spotlight SAR (SA-BSSAR) has attracted attention recently due to its flexibility. This letter proposes a new frequency synchronization method and a 2-D frequency domain imaging algorithm for SA-BSSAR. In this letter, the frequency error due to the splitting of receiver and transmitter is first obtained from the direct-path signal by using the fractional Fourier transform (FRFT), and then the spectral analysis (SPECAN) technology is used to eliminate Doppler aliasing because of using low pulse repetition rate (PRF). Based on this, we deduce the echo signal 2-D spectrum via the method of series reversion (MSR) and analyze the range-variant under SA-BSSAR. Finally, decouple the range frequency domain from the range-variant term by chirp- $z$ transform (CZT) to achieve scene focusing imaging. All the proposed algorithms are implemented by FFT without interpolation, which ensures efficiency. The final experimental results show that the algorithm can efficiently compensate for the frequency error, eliminate the Doppler aliasing and correct the range cell migration (RCM).