The clutter suppression of space-time adaptive processing (STAP) will degrade severely in forward-looking airborne radar if the range-dependence of clutter spectrum is not compensated correctly. In this paper, a new method for mitigating the range-dependence of the clutter spectrum based on the geometry-aided subspace projection is proposed. In the proposed method, the clutter covariance matrix of the range under test is firstly constructed based on the prior knowledge of antenna array configuration, and then the clutter subspace projection matrix is obtained by decomposing the corresponding covariance matrix. In the following, the mitigation of the range-dependence is accomplished by applying this clutter subspace projection matrix to transform the secondary range samples to obtain an approximate clutter subspace with the range under test. The simulation results show that the proposed method can mitigate the range-dependence significantly.