Antenna beam steering along azimuth of sliding spotlight SAR(synthetic aperture radar) system was accomplished by step steering scan in practical operation. However, this scanning method may cause the paired-echo in the image of sliding spotlight SAR. To analyze the reason of pairedecho, the step steering scan model of sliding spotlight SAR was established. And the relationship in quantitatively between beam stepped steering and both amplitude and position of paired-echo by Tyler series expanding and Fourier series were calculated out. The theory analysis and signal simulation indicate that the model demonstrates the phenomenon well, and the model provides the theory basis for the design of the sliding spotlight SAR beam steering scan parameters.