Introduction: Low birth weight infants (LBWI) frequently require peripherally inserted central catheter (PICC) placement for prolonged parenteral nutrition. Catheter tip malposition remains a major cause of catheter-related complications. Accurate prediction of insertion length is therefore critical. However, traditional surface measurement methods demonstrate limited precision in LBWI, and existing predictive models often fail to account for variations in puncture sites, limb laterality , and body positioning. Objective: To evaluate the association between standardized PICC insertion length and anthropometric parameters in LBWI and to develop site-specific predictive equations. Methods: In this retrospective study , 175 LBWI who underwent PICC placement in a tertiay NICU between January 2023 and December 2024 were included. Pearson correlation analysis was performed to examine relationships between anthropometric variables and standardized insertion length. Variables with significant correlations were entered into linear regression models to establish predictive equations for each venous access . Agreement between predicted and actual insertion lengths was assessed using Bland–Altman analysis. Results: Standardized insertion lengths of the right basilic, right median cubital, right cephalic, and right great saphenous veins were positively correlated with arm circumference, leg circumference, and body weight (all P < 0.05), with body weight demonstrating the strongest correlation. Weight-based regression models were established for all four venous sites. Bland– Altman analysis showed small mean differences and acceptable limits of agreement. Conclusions: Body weight is an independent predictor of standardized PICC insertion length in LBWI. Weight-based equations provide statistically reliable estimates for clinical application.