Abstract: Using the theory of synthetic aperture and atmosphere refractivity, the formulas of the azimuth resolution atmospheric error for spaceborne synthetic aperture radar are derived in this report. Using ionospheric electron density profiles and the lower atmospheric refractivity profiles of different time and places, the azimuth resolution is calculated for different frequencies. Simulation results are discussed and some conclusions are summarized that the atmospheric error of azimuth resolution is at millimeter level, and the absolute value will be reduced as frequency increases.