MIMO-SAR （ multiple input multiple output synthetic aperture radar） is a new kind of SAR s y stem， which combines multiple transmitters and multiple receivers to acquire mo r e equivalent phase centers and more degrees of freedom．By this way， MIMO S AR ex te nds system working mode and improves the performance．However, SAR echo is scatt er ed from a distributed scene. Cross-correlation noise will overlap and interfere n ces will degrade the image. That will make the separation methods used in MIMO r adar fail in MIMO-SAR. Therefore, separation is a key problem to the implementa tion of MIMO SAR. This paper introduces the principle of MIMO SAR and it s ap plications, then reviews the methods proposed in current literatures．The princi p les and constrains of every separation method are pointed out. Some new approach es t o the problem are proposed and discussed．