This paper compared the Doppler frequency and azimuth frequency in detail during the researching of a representative bistatic SAR imaging theory and found that using Doppler frequency instead of azimuth frequency in the general SAR processing with traditional imaging algorithms has little influence on the azimuth resolution of the fine imaging , the error is less than 4%. But if the differences between the two frequencies are neglected in the condition of larger squint angle, bigger transmit signal chirp rate, and longer wave length, the azimuth resolution error may reach as much as 10% even 27%. So, whether Doppler frequency can be used instead of azimuth frequency depends on the SAR systems with different specific parameters and the different requirements in specific imaging precision.