Abstract: This paper focuses on the problem of clutter rank estimation of coherent airborne MIMO radar with arbitrary transmitted waveform synthetic structures. A new construct method of clutter rank estimation for Multiple Input Multiple Output (MIMO) radar is presented. Clutter rank can be estimated by an equivalence matrix which is constructed by the waveform synthetic structures and sparse structure from the method, instead of the computation complicated direct decomposition of the Clutter Covariance Matrix (CCM). A simple and effective estimation rule of clutter rank is proposed and strict proofed based on the decomposition of the construct matrix. The quantitative relationship among the synthetic structures, sparse structure and Degree Of Freedom (DOF) of clutter is established, and the eigenspectrum structure of the clutter under arbitrary transmitted waveform synthetic structures can be obtained from the method and rule. Simulations also verify the accuracy of the method and rule. The proposed theory can be used to design an optimal synthetic structure and effective STAP algorithm.