Abstract: Sea surface current fields are important oceanic and climatic variables. Due to its capability for global coverage and direct observation of sub-mesoscale sea surface current fields, the Doppler scatterometer has become a frontier in ocean remote sensing technology research. The calibration and quantitative measurement of the Doppler scatterometer are the foundations and prerequisites for current field observations, as well as a critical core issue that needs to be addressed. This paper develops a ground-scatter-echo simulation model and uses it to simulate and analyze the Doppler spectra of natural extended targets that could potentially serve for Doppler scatterometer calibration. First, the variation characteristics of the Doppler spectrum were compared with different platform velocities, incidence angles, and azimuth angles. The results indicate that platform motion speed is the primary factor affecting Doppler spectrum characteristics, while variations in incident and azimuth angles also have significant impacts. Then, the Doppler spectral characteristics of ground extended targets were analyzed with different terrain-undulation conditions, namely varying height variation and central height. Simulation results indicate that targets with larger height variation exhibit greater Doppler spectral frequency shifts, whereas changes in central height have little effect on the Doppler spectral characteristics. Therefore, in the selection of calibration targets, relatively flat extended targets should be chosen, while the absolute elevation of the target is not a critical factor. Finally, an analysis of the ground echo Doppler frequency shift was conducted and validated using the DEM model. The findings of this study will provide support for further research on Doppler scatterometer calibration.