Abstract: In spotlight SAR system design, PRF selection is very important. The article introduces the limitations of PRF selection and pointed that because there are essential differences between strip-map SAR and spotlight SAR, the traditional method to analyze the azimuth ambiguity in strip-map SAR does not match the situation in spotlight mode. The article introduces the calculation function of spotlight SAR in details, and expounds that in spotlight SAR system design, on the base of satisfying basic limitations, using the simulation curves of azimuth ambiguities can optimize PRF selection. Simulation results indicate that this optimization is very important for spotlight SAR system design.