Antidromic atrioventricular re-entrant tachycardia (AVRT) is a rare clinically wide-QRS tachycardia that can often be misdiagnosed as ventricular tachycardia (VT). If the accessory pathway (AP) is slower than the normal atrioventricular (AV) junction, evidence of preexcitation will not appear on the surface electrocardiogram (ECG), which increases the difficulty of diagnosis. This report describes a 13-year-old male patient who was diagnosed with VT at a local hospital at the time of onset of palpitation. ECG revealed wide-QRS-complex tachycardia, and the ECG after the termination of palpitations was within the normal range of sinus rhythm. The patient was confirmed to have an AV accessory pathway on the left anterior free wall with Mahaim fiber features by intracardiac electrophysiological study at our hospital. Wide-QRS-complex tachycardia was confirmed to be antidromic AVRT. We treated the patient with catheter radiofrequency ablation, which eliminated tachycardia. By comparing the ECGs before and after the operation, we observed the hidden manifestation of the ventricular preexcitation wave. There was no recurrence after half a year of follow-up.