The invention discloses a high-precision amplitude modulating method to receive synthetic aperture radar signal based on automatic testing, which comprises the following steps: constructing automatic testing system with signal source, source scaler, programmable power meter and communication control device; obtaining the relationship curve of signal power output by voltage-controlled waveshape code and source scaler; obtaining the modulated waveshape code to compensate the error of the source scaler amplitude according to the curve and SAR system parameter. The invention satisfies the precision modulating need for synthetic aperture radar signal, which reduces the clutter side lobe through compensating non-linear property in the source scaler according to the fitted modulating waveshape code of SAR system parameter.