Pulse transit time (PTT) based continuous cuff-less blood pressure (BP) monitoring has attracted wide interests owing to its potential in improving the control and early prevention for cardiovascular diseases. However, it is still impractical in large-scale clinical application due to the concern of BP measurement accuracy. Since such approach strongly relies on the PTT-BP model under certain theoretical assumptions, the accuracy would be affected by the vessel properties alterations induced by cardiovascular disorders. Atrial fibrillation (AF) is one of the most common cardiac diseases which often coexist with hypertension. The present study sought to examine the Impact of AF on the PTT and BP, validate the capability of PTT based cuff-less methods on AF patients. By investigating the PTT and BP on 74 critically ill patients with AF, we found that parameters including PTT, R-R interval and diastolic BP (DBP) were significantly changed when AF occurs, while the systolic BP (SBP) value and photoplethysmography intensity ratio (PIR) changed little. Further, by performing two cuff-less BP estimation method, we found that the estimated accuracy is decreased on PTT based method when AF occurs, but there is little change on PIR based method. The findings demonstrated that the impact of AF on PTT is significant, which would also influence the PTT-BP relationship. But the PIR would still be a predictive factor for BP estimation for AF patients.