BACKGROUND: Atrial fibrillation (AF) poses significant challenges for coronary CT angiography (CCTA) due to R-R interval variability, often necessitating high radiation dose protocols. Absolute phase gating, which uses fixed millisecond delays rather than percentage-based cardiac cycle timing, may optimize image quality while reducing radiation exposure. PURPOSE: To evaluate whether absolute phase gating reduces radiation dose while maintaining diagnostic accuracy and image quality compared to conventional relative phase gating in patients with AF undergoing CCTA. METHODS: This retrospective matched cohort study included 280 consecutive patients with AF (140 per group) who underwent CCTA between January 2021 and December 2023. Patients were matched by heart rate, BMI, and calcium score. The absolute phase gating group used fixed 300-millisecond post-R-wave triggering; the relative phase gating group used conventional 75% R-R interval triggering. Primary endpoints were diagnostic accuracy (sensitivity, specificity) for ≥50% stenosis and assessable segment ratio. Secondary endpoints included radiation dose parameters and image quality scores. Invasive coronary angiography served as reference standard in 118 patients (42.14%). RESULTS: Radiation dose was 64.28% lower with absolute phase gating (median DLP: 187.50 vs. 524.80 mGy·cm, p < 0.001; median effective dose: 2.63 vs. 7.35 mSv, p < 0.001). Diagnostic accuracy remained comparable (sensitivity: 96.61% vs. 94.74%, p = 0.524; specificity: 89.47% vs. 86.67%, p = 0.612). Assessable segment ratio significantly improved (99.05% vs. 95.71%, p < 0.001). Mean image quality scores were higher with absolute gating (3.21 ± 0.58 vs. 2.84 ± 0.71, p < 0.001). CONCLUSIONS: In patients with AF undergoing CCTA, absolute phase gating can potentially reduce radiation dose while maintaining or improving diagnostic performance relative to the conventional relative phase gating protocol used at our institution. Because the magnitude of any dose reduction depends in part on the acquisition window and dose-modulation settings of the compared protocols, the benefit should be interpreted in the context of the specific comparator. These findings are consistent with the Image Wisely initiative for responsible patient radiation dose management.