Abstract: Objective: To analyze the characteristics of brain electrical activity detected by amplitude-integrated electroencephalogram(aEEG) in preterm infants with different gestational ages.Methods: One hundred and thirty-five preterm infants with gestational age 27-36⁺⁶ weeks were divided into the <28 weeks,28-29⁺⁶ weeks,30-31⁺⁶ weeks,32-33⁺⁶ weeks and 34-36⁺⁶ weeks groups according to the gestational age.The first aEEG monitoring was performed for 8 h in all cases after 3 to 5 days of birth.The changes of voltage continuity,upper and lower borders,and sleep-wake cycle were analyzed.Results: Among the preterm infants with gestational age <28 weeks,the discontinuous voltage was found,and periodic changes in sleep was not found.With the increasing of gestational age,the ratio of continuous voltage and sleep periodicity gradually increased(P<0.01).With the increasing of gestational age,the upper amplitude value of active sleep period,lower amplitude value of active sleep period and course of one sleep cycle gradually decreased,increased and shortened,respectively(P<0.01).Conclusions: The aEEG can better reflect the brain maturation of preterm infants,and the EEG background activity in the preterm infants with older gestational age is more mature.The aEEG has a better assessment value in brain function prognosis of preterm infants.