For the Forward-Looking (FL) radar using a linear Frequency Diverse Array (FDA), this paper ,based on the space-time signal model, particularly studies features of clutter space-time spectrum spread and phase range walk. The feature of clutter spectrum spread represents the clutter spectrum spread in angle-Doppler space. When the stepped-frequency is lower, the number of array elements and pulse are a few, clutter space-time spectrum spread is not evident. The feature of phase range walk just relates to the range, stepped-frequency and pulse repetition frequency; the effect of phase range walk leads to the separateness of range ambiguous clutter spectrum, which benefits the suppression of range ambiguous clutter. The validity of the analysis in this paper is proved with simulation results.