A coronary artery aneurysm (CAA) occurs when the wall of the coronary artery that surrounds and nourishes the heart becomes locally dilated. Coronary aneurysms are asymptomatic; however, a serious complication is myocardial infarction caused by thrombosis, which can lead to sudden death. Computational fluid dynamics analysis is used to evaluate the risk of thrombosis in CAA. However, a rigid wall model is generally assumed; thus, this analysis does not accurately reproduce the hemodynamics of coronary arteries adjacent to the heart. This study investigated the effect of wall elasticity on the hemodynamics of CAA caused by Kawasaki disease. A fluid-structure interaction (FSI) analysis considering the heartbeat was performed to compare the time-averaged wall shear stress (TAWSS), highly oscillatory low magnitude shear (HOLMES), and oscillatory shear index (OSI) using a rigid wall model. A computational model of a branching coronary artery with an aneurysm was created from the vessel model repository provided by the open source cardiovascular modelling platform SimVascular, with a vessel wall thickness of 0.9 mm. For the boundary conditions, a physiological flow rate waveform was imposed at the inlet. The distal branches at the outlet were connected to a lumped parameter model considering the intramyocardial pressure. As a result, considering elastic walls, the OSI in the CAA increased by 42.8%. In contrast, the TAWSS decreased by 3.6%, and the HOLMES showed only a 5.4% reduction. These findings suggest that elastic wall properties significantly impact the OSI, indicating that simulations assuming rigid vessel walls may overlook the progression of atherosclerosis.