Objective To explore the relationship between fluid balance during the first 14 postnatal days and critical clinical complications in extremely low birth weight infants (ELBWIs), with the aim of reducing the impact of fluid overload on adverse outcomes. Methods Clinical data of 226 ELBWIs admitted to the neonatal intensive care unit of the Affiliated Obstetrics and Gynecology Hospital of Nanjing Medical University from January 2019 to December 2024 were retrospectively analyzed. Infants who developed severe bronchopulmonary dysplasia (sBPD) or died were classified as the critical complication group (n=99), and survivors with at most moderate BPD (none, mild, or moderate) were classified as the control group (n=127). Fluid balance indices were defined by the percentage change from birth weight. The association between fluid overload during the first 14 postnatal days and the occurrence of sBPD or death was evaluated. Results Univariate analysis showed that fluid balance on postnatal days 3, 7, 10, and 14, as well as weight loss <3% within the first 3 days and <6% within the first 7 days, was significantly associated with the occurrence of critical complications (all P<0.05). After adjustment for confounders in multivariable logistic regression, fluid overload on postnatal days 3, 7, 10, and 14 was a risk factor for critical complications (OR>1, P<0.05), and the risk of critical complications increased significantly when weight loss was <3% within the first 3 days (OR=5.213, 95%CI: 2.349-11.567) and <6% within the first 7 days (OR=3.920, 95%CI: 1.806-8.511). Conclusions In ELBWIs, early fluid overload may be associated with an increased risk of sBPD or death, and moderate fluid restriction may help improve prognosis; prospective studies are required for further validation.