Abstract: Detection of fast air moving targets from high speed platform is challenged by the range walk of both the clutter and moving targets. It is well known that keystone formatting can be used to compensate for the range walk of multiple moving targets simultaneously without using the knowledge of the motion parameters. However, in the presence of serious Doppler ambiguity of fast air moving targets, distribution of the clutter will be affected by the keystone formatting matched to the ambiguity number of targets, and as a result the space-time adaptive processing (STAP) performance will degrade. The effects of keystone formatting on the clutter distributions and on the performance of STAP are studied in this paper. We concluded that keystone formatting can compensate the range walk of both the target and clutter when the target is ambiguity-free. However, when the target is Doppler ambiguous, range walk compensation of the target by keystone formatting induces new range walk to the clutter, which will affect the distribution of the clutter and further degrade the STAP performance. The above conclusions are helpful in deriving better methods for the detection of fast air moving targets on high speed platform.