Synthetic aperture ladar can work with low pulse repetition frequency and obtain wide scene with muti-channel in terrain observation by progressive scans mode.However,the frequency spectrum in azimuth of the raw data in each channel is ambiguous,which ambiguity can not be removed by conventional spactial beamforming.To solve this problem,a novel method of spactial beamforming based on spectral compressing was proposed.Firstly,a transformation in azimuth was introduced with deramp technology.Then,spactial beamforming was used to remove the ambiguity.Thirdly,a phase factor was used to compensate the phase error induced by the deramp.Finally,a conventional imaging algorithm can be used to abtain images from unambiguous data.The imaging experiment with numerical data validates the feasibility and effectiveness of the proposed method.