Background: Intracavity electrocardiogram (IC-ECG) guidance is an effective technology for monitoring the tip locations of centrally inserted central catheters (CICCs) and peripherally inserted central catheters (PICCs). However, for patients with an indwelling CICC, there has been no study on whether the CICC affects PICC tip positioning with IC-ECG guidance. Methods: Thirty people with a CICC requiring PICC insertion, matched with age to controls without a CICC. The stability of IC-ECG waveforms, the amplitude of P-waves, and the accuracy of PICC tip location were compared. Results: There was no significant difference in the stability of the IC-ECG waveforms (χ² = 0.22, p = 0.64). The amplitudes of baseline P-waves and ideal P-waves also showed no significant difference ( t1 = 0.06, p1 = 0.96, t2 = 0.80, p2 = 0.43). Neither the accuracy of tip location (χ² = 1.40, p = 0.50) nor the distance of PICC tip ( t = −0.03, p = 0.98) were significantly different. Conclusion: For patients with an indwelling CICC, the position of PICC tip can be accurately determined by the dynamic changes in the P-wave amplitude.