The case of a patient in whom hemodynamic and electrocardiographic studies using the occlusion test for coronary artery fistulas (CAF) were safely performed prior to catheter embolization is reported. A 1-year-old girl had a separate right coronary artery arising from a left single coronary artery that formed a significant coronary artery fistula to the right ventricle. Coronary steal by the large coronary artery fistula narrowed the left coronary artery. The right coronary artery branches could not be clearly identified due to an overlap with the fistula. Due to the long porous CAF, embolic procedures could cause serious complications. We confirmed the safety by performing an occlusion test of the CAF's proximal blood vessels. Following total occlusion of the CAF for 10 minutes, pulmonary arterial pressure and aortic blood pressure were not significantly changed. No bradycardia, atrioventricular block, or ST changes were observed. Coil embolization treatment was performed safely. For patients with long distal CAF complicated with a single coronary artery, myocardial ischemia and conduction system disorders can be identified by performing the occlusion test before embolization.