Objective To explore the short-term efficacy and tolerance of active parenteral nutrition support protocol (high initial dose of amino acids and fat emulsion) in preterm infants with gestational age < 34 weeks. Methods A total of 138 preterm infants with gestational age < 34 weeks who were admitted to hospital within 24 hours after birth from May 2019 to December 2019 were randomly divided into two groups according to different amounts of amino acids and fat emulsions applied in the early period. In the active parenteral nutrition group (n= 69 ), the dosage of amino acids started from 2 . 5 g/(kg · d), increased by 1 . 0 g/(kg · d) daily, and maintained after reaching 4 g/(kg · d); the dosage of fat emulsion started from 2 . 0 g/(kg · d), increased by 0 . 5 ~ 1 . 0 g/(kg · d) daily, and maintained after reaching 3 . 5 g/(kg · d). In the routine parenteral nutrition group (n= 69 ), the dosage of amino acids started from 2 g/(kg · d), increased by 0 . 5 g/(kg · d) daily, and the final dosage was no more than 4 g/(kg · d); the dosage of fat emulsion started from 1 g/(kg · d) with final dosage ≤3 g/(kg · d), and this final dosage was maintained. Other routine enteral and parenteral nutrition support programs were implemented at the same time. Results The weight gains of preterm infants in the active parenteral nutrition group were higher than those in the routine parenteral nutrition group in the 1 , 2 and 3 weeks after birth, and the parenteral nutrition time and the days used to achieve total enteral feeding were shorter than those in the routine parenteral nutrition group. All the differences were statistically significant (all P< 0 . 05 ). The rates of lactose intolerance and apnea in the active parenteral nutrition group were lower than those in the routine parenteral nutrition group, with statistical significance (P< 0 . 05 ). Only the blood urea nitrogen level at 24 hours postnatal and glutanine aminotransferase level at 2 weeks postnatal had statistically significant differences between the two groups (P< 0 . 05 ). The above indexes in the active parenteral nutrition group were lower than those in the routine parenteral nutrition group. Conclusion It is safe and effective to give active parenteral nutrition support (high dose of amino acids and fat emulsion) within 24 hours after birth to preterm infants with gestational age < 34 weeks, It can significantly reduce the duration of parenteral nutrition and promote the short-term weight gain without increasing complications.