Adenine base editors (ABE) are genome-editing tools that have been harnessed to introduce precise A•T to G•C conversion. However, the low activity of ABE at certain sites remains a major bottleneck that precludes efficacious applications. Here, to address it, we develop a directional screening system in human cells to evolve the deaminase component of the ABE, and identify three high-activity NG-ABEmax variants: NG-ABEmax-SGK (R101S/D139G/E140K), NG-ABEmax-R (Q154R) and NG-ABEmax-K (N127K). With further engineering, we create a consolidated variant [NG-ABEmax-KR (N127K/Q154R)] which exhibit superior editing activity both in human cells and in mouse disease models, compared to the original NG-ABEmax. We also find that NG-ABEmax-KR efficiently introduce natural mutations in gamma globin gene promoters with more than four-fold increase in editing activity. This work provides a broadly applicable, rapidly deployable platform to directionally screen and evolve user-specified traits in base editors that extend beyond augmented editing activity.