Abstract: Each of the elements in orthogonal MIMO radar transmits orthogonal waveform. The various emitted signals can not be combined to form a focused beam. The dynamic range of returned signal reflected by a target will be smaller than that of the traditional phased array radar, and so it is advantaged for MIMO radar to detect weak targets in strong clutter. The dynamic range is mainly dependent on the clutter power. In this paper, the signal power of MIMO radar using OFDM-LFM waveform is analyzed, and optimized random initial phases are utilized to reduce the signal instant power and to modify the dynamic range of returned signal. The  improvement of the performance of detecting weak target is showed through simulation by comparing MIMO radar to traditional phased array radar