Space-time adaptive processing (STAP) is an effective method for moving target detection in airborne radar. However, fast velocity of air target induces serious range walk and Doppler ambiguity. To make things worse, acceleration of the target will result in Doppler spread. Due to the range walk and Doppler spread of maneuvering air target, the performance of the conventional STAP degrades dramatically. To detect the above mentioned fast air maneuvering targets, a new method is proposed in this paper, which removes the clutter before keystone formatting, hence the effects of keystone formatting for the ambiguity targets’ range walk compensation on the clutter distributions and further on the performance of STAP are avoided. Doppler spread of the target is compensated by the estimated acceleration by a modified optimum adaptive processor. Therefore, good detection performance of fast air maneuvering targets can be achieved. Effectiveness of the new method is verified via simulation examples.