Modelling and characteristics analysis of two-dimensional space-time clutter for airborne MIMO radar are the key precondition for the design of airborne MIMO Space Time Adaptive Processing (STAP) algorithm. The two-dimensional space-time clutter antenna elements model and vector model are founded based on the geometric characteristics of airborne MIMO radar in this paper. The estimating method of clutter covariance matrix is researched under the condition of range ambiguities. The simulation results show that the airborne MIMO radar clutter is good in two-dimensional space-time diagonally distributing characteristics. Compared with the phased array radar, MIMO radar is wider in detecting angle of vision and greater in freedom of system, by which it can easily suppress clutters.