Phenylketonuria (PKU) is a common disease associated with amino acid metabolism, and usually occurs in newborns. It can cause serious neurological diseases and even death. However, owing to inadequate-effective treatment, it can only be slowed by a low-phenylalanine (Phe) diet. In addition, PKU screening is essential for newborns in many countries. Therefore, rapid screening is crucial for preventing damage and meeting the large sample diagnosis demand. For confirmed patients, a convenient method to monitor their regular Phe levels is required. However, current clinical methods do not meet the rapid screening and convenient monitoring requirements. Herein, a rapid and facile electrochemical device based on platinum-doped reduced graphene oxide nanocomposites was developed to detect PKU biomarker-Phe. The results demonstrated that the developed electrode has great sensitivity, selectivity, and stability. The detection range was 0.0001 mM to 6 mM with a limit of detection of 0.01 μM. Therefore, this work offers a simple and rapid method for point-of-care PKU screening and daily monitoring.