Primary monosymptomatic nocturnal enuresis (PMNE) is one of the most common urinary system problems in childhood. The exact pathological mechanism has not been fully elucidated yet, but excessive nocturnal urination is considered one of the core pathogenic mechanisms. Studies have found that a considerable number of PMNE patients have abnormal secretion of arginine vasopressin (AVP) at night. Currently, the "gold standard" for diagnosing nocturnal polyuria is through a voiding diary, which is cumbersome and the records may contain errors, and there are many inconveniences in clinical implementation. Therefore, finding objective and simple biomarkers to assist in diagnosis and classification is a current clinical research hotspot. Copeptin is the C-terminal fragment of the precursor protein of AVP and is released into the blood simultaneously at the same molar ratio as AVP, making it an ideal alternative biomarker for AVP. This study aims to systematically and deeply explore the diagnostic value of serum copeptin in PMNE, especially in its different subtypes (NP-PMNE vs. NNP-PMNE), analyze its correlation with clinical severity, and explore more precise detection strategies, in order to provide new and objective biological tools for the clinical management of PMNE.