Abstract: The seriously expended clutter Doppler and finite sample supports are the problem of high velocity radar platform, and the performance of conventional Space Time Adaptive Processing (STAP) will degrade due to covariance matrix error or inaccurate target constraint. A research on clutter Degree of Freedom (DoF) of forward-looking radar is discussed. The eigenvalue spectrum of clutter covariance matrix is relative with aperture bandwidth product, time-width bandwidth product together with the correlation of spatial and temporal frequency. Robust STAP method based on space-time multi-beam transforming is proposed. First, the DoF of clutter is reduced through space-time multi-beam transforming. Then, the extended Doppler constrain method is used to improve the performance and robustness of the processer. The numerical simulations show the effectiveness of the proposed method. Even when the training data contains the target signal, the proposed robust method can maintain up to 5 dB Signal-to-Clutter-plus-Noise Ratio (SCNR) improvement compared with the conventional methods.