Compared with phased array radar, Multiple-input multiple-output (MIMO) radar also needs to consider the transmit degree of freedom (DOF). Moreover, the missile-borne platform is usually a forward-looking planar array, so the signal form is very complex. The extended Brennan's rule under traditional airborne MIMO radar is difficult to estimate the DOF of clutter under the missile-borne platform. Focusing on the clutter DOF estimation for missile-borne MIMO radar, this paper proposes a fast clutter DOF estimation method. First, by using the characteristics of virtual array of MIMO radar in the airspace, the transmitting and receiving arrays are convoluted to obtain the synthetic virtual arrays. Then, simplified space-time steering vectors are obtained based on the virtual array, the clutter signal reduction dimension and clutter DOF estimation can be achieved. The results of the simulation experiments verify the effectiveness of the proposed method.