Distal false lumen (FL) expansion is a common complication after thoracic endovascular aortic repair (TEVAR) of type-B aortic dissection (TBAD). FL expansion is likely to cause post dissection aortic aneurysm (PDAA). At present, the biomechanical mechanism leading to the expansion of the FL is not clear, resulting in difficulties in its prevention and treatment. This paper presents a patient-specific one-way fluid–structure interaction (FSI) method for post-TEVAR TBAD patients. This method was then employed to predict the hemodynamic parameters and wall stress in five unstable and five stable patients post-TEVAR. Simulation results were employed to identify the characteristic mechanical parameters for the FL expansion, and to propose a possible mechanism behind FL expansion involving the relationship between the aortic morphology and the characteristic parameters. The pressure difference between false and true lumen, and the average wall stress of FL are recognized as the characteristic parameters for FL expansion, which effectively differentiate the two groups. The threshold value of wall stress between the two groups is about 75 kPa. Abnormally high luminal pressure difference and wall stress in the unstable group were attributed to their anatomical features, such as:​ enlarged FL, compressed TL, elevated thrombus ratio in FL, and significantly larger tear size. In conclusion, post-TEVAR expansion of the FL is correlated to high luminal pressure difference and elevated FL wall stress, which might be caused by the morphological features of the aorta.