Abstract Dechirp technology for LFM (liner frequency modulation) echo signal will cause sidelobe. The sidelobe of a strong scattering object may mask weak scattering targets nearby. By suppressing the sidelobe in the specified location with a calculated window function, the sidelobe masking can be eliminated. The mainlobe level and the sidelobe level in the specified location are used to construct the objective function. By iterating, we can suppress the sidelobe in the specified location without mainlobe attenuation. The numerical simulations and the actual measurements demonstrate that the proposed method can suppress sidelobe successfully and eliminate the sidelobe masking effectively without mainlobe attenuatio.