In this study, a novel method for overcomplete dictionary construction for sparse recovery (SR) space-time adaptive processing (STAP) is proposed, which can effectively mitigate the off-grid effect. The proposed method utilises the clutter ridge, calculated via some prior knowledge such as radar system parameters and modern inertial navigation system information, to discretise the grids in the angle-Doppler plane. The authors also investigate the accuracy of clutter ridge in the presence of several typical non-ideal factors such as array errors, intrinsic clutter motion and aircraft crab. In particular, a parameter is introduced to adaptively adjust the grid interval to avoid the strong column coherence of the dictionary. The simulation results show that the signal-to-clutter-plus-noise ratio for the SR STAP with the proposed dictionary is significantly improved, compared with the one utilising conventional dictionary. Moreover, it is shown that the novel method not only works more robustly for the side-looking array, but also has better performance for other array orientations.