Objective To investigate the dynamic changes in regional cerebral oxygen saturation (rScO₂) variability within the first week after birth in preterm infants and its relationship with different types of brain injury. Methods A retrospective study included 70 preterm infants with a gestational age of 28-32 weeks admitted to Cangzhou People's Hospital from August 2021 to June 2025. All infants underwent continuous near-infrared spectroscopy (NIRS) monitoring of rScO₂ within the first 7 days after birth. Based on cranial ultrasonography and magnetic resonance imaging findings, infants were divided into intraventricular hemorrhage (IVH) group (n=23), periventricular leukomalacia (PVL) group (n=18), hypoxic-ischemic encephalopathy (HIE) group (n=11), and control group (n=18). The coefficient of variation (CV), slope, peak, and trough values of rScO₂ were calculated across different time windows, and their correlations with brain injury type and severity were analyzed. Results Within the first 7 days after birth, rScO₂ CV showed a biphasic pattern characterized by an initial increase, followed by a decrease, then a second increase and decrease. The first high-variability period occurred at 0 to <24 hours, and the second at 72 to <120 hours. The CV at 6 hours after birth was positively correlated with brain injury severity (r_s =0.72, P<0.001). The area under the receiver operating characteristic curve (AUC) for diagnosing brain injury using 6-hour CV was 0.862 (P<0.05). A combined predictive model incorporating CV values at 6, 24, and 72 hours achieved an AUC of 0.924 (P<0.05). Conclusions The variability of rScO₂ within the first week after birth in preterm infants exhibits a biphasic fluctuation pattern. Different brain injury types show characteristic variability patterns, and early CV values can serve as sensitive indicators for early identification of brain injury.