In order to obtain the law of Doppler spectrum distribution in the area radiated by the antennas mainlobe when the radar altimeter moves relative to the ground, a wide-band Doppler echo model is established, and the beam irradiation area is divided into several rectangular scattering grids by using the grid mapping method based on the equal Doppler band and the equal distance ring.The simulation of echo Doppler power spectrum is conducted by using the method of echo power superposition of each rectangular grid in the equal Doppler band.It overcomes the shortcomings of complicated calculation based on the polar-coordinate integration method, which is not conducive to the subsequent engineering realization.The simulation results are consistent with the theoretical values, which shows that the model is reasonable.Finally, the law of Doppler spectrum distribution of the radar altimeter at different altitudes and speeds is obtained, which provides a theoretical support for the subsequent echo simulation of the radar altimeter.