Objective To explore the feasibility of intracavitary electrocardiogram (IC-ECG) for identifying catheter tip of upper arm port during indwelling period. Methods A total of 151 gynecologic cancer patients with upper arm port were recruited by conve-nience sampling method. Routine surface ECG (SECG) and IC-ECG were performed to identify the changes in P wave. Different changes in P wave indicated different positioning of the catheter tip. Finally, all patients received chest X-ray examination to determine the actual position of the catheter tip:ideal position, deep position, and shallow position. The cut-off values and diagnostic efficacy of IC-ECG parameters were analyzed. Results The accuracy of catheter tip location predicted by changes in P wave was 80.8%. IC-ECG P wave amplitude to SECG P wave amplitude ratio (IP/SP) could be an effective indicator to distinguish between ideal position and deep position, and also between ideal position and shallow position (area under the receiver operating curve=0.721 and 0.643, respectively), and the cut-off value of IP/SP was in the interval of 1.820-2.230. Conclusion IC-ECG can be used to determine the catheter tip of upper arm port during the indwelling period for its high diagnostic accuracy, which may facilitate to ensure the safe indwelling of upper arm port.