Abstract: ‍ ‍In a high pulse-repetitive-frequency forward-looking array radar， the clutter is not only range-dependent but also range-ambiguous， which leads to a sharp drop in the clutter suppression performance of the traditional space-time adaptive processing method or even failure. Focusing on the range-ambiguous clutter suppression problem of forward-looking array radar， this paper proposes a range-ambiguous clutter suppression method using the controllable degree-of-freedom provided by the FDA-MIMO radar. This method separates the clutter in the unambiguous area and the ambiguous area in the spatial frequency domain， and uses the method of Tangent Derivative Based Updating （Tangent DBU， TDBU） to compensate for the range-dependence under the condition of the forward-looking array. Finally， space-time adaptive signal processing is performed. This method effectively solves the problem of range-ambiguous clutter suppression for forward-looking array radar， and improves the clutter suppression performance. Simulation results demonstrate the effectiveness of proposed method.