Polyetheretherketone (PEEK) and its short fiber-reinforced variants (SFR-PEEK) find extensive applications across aerospace, medical, and electronic sectors owing to their excellent material characteristics. However, the heterogeneous characteristics of fiber-reinforced materials pose challenges during grinding processes, presenting distinct behaviors compared to the PEEK matrix composites. This study conducts single abrasive grain scratching experiments to understand scratching behavior and deformation mechanisms, along with analysis of chip formation mechanisms. Results reveal that as scratching depth increases, pure PEEK exhibits evident scratch grooves with plastic bulging, indicative of typical plastic material scratch deformation. Quantitative analysis shows that the maximum scratch depth of CF30/PEEK or GF30/PEEK is about 18%∼22% smaller than that of CF10/PEEK under identical loads. Conversely, in SFR-PEEK, scratches are confined to the PEEK matrix surface, suggesting that increased fiber content enhances the material’s resistance to deformation. Additionally, an average stress gradient model was established using the stress theory of single grit scratching. Combining with material’s chip characteristics, the critical chip formation depth (CCFD) of PEEK and SFR-PEEK was predicted to be 130 nm, with a prediction error of less than 8.5%. This study provides theoretical guidance for the industrial production of ultra-precision grinding of PEEK and SFR-PEEK.