To solve the problem of clutter suppression faced by airborned MIMO radar,after considering the element error,channel uncertainty as well as clutter fluctuation, , the clutter model for airborne MIMO radar under different error conditions is established,a new space-time adaptive processing method based on MIMO systems——M3-CAP method is proposed. Firstly,temporal filtering is performed on the space-time received data,and then the output data of three Doppler channels which are the detected Doppler channel and two of its adjacent channels,is adaptively filtered .By combining MIMO systems and 3-CAP method effectively,the advantage of waveform diversity is extended into 3-CAP method,this new method improves the degree of freedom and clutter estimation accuracy. By analyzing and comparing the clutter suppression performance under different radar systems and different error conditions,the result turns out:the clutter suppression performance of M3-CAP method is superior to JDL-GMB、Modified JDL、3-CAP methods.