Abstract: Spectrum Modification is a simple, effective method of suppressing pulse compression sidelobe, which can suppress the fluctuation in subbands via modifying spectrum and control the pulse-compressed range sidelobe through windowed frequency domain. In the research of MIMO radar, the orthogonal signals are compressed by pulse ,then undesired range sidelobe is produced, this method cannot suppress the range sidelobe in an effective way if directly-applied in pulse compression of each subband because subbands are mutually cohered. To solve this problem, this essay provides a method via modifying the spectrum and adding window to frequency domain of composite MIMO radar signal, which would suppress pulse compression sidelobe more effectively. And the simulation also ends up with a ratio of mainlobe to sidelobe more than 40dB, as well as a less complex system and reduced computation. In the case that the deviation of Doppler and angle is not large, the peak sidelobe level is lower by using this method, contrast with the traditional matched filter result.