Patients will be randomized to early PN group or late PN group at day 3 after abdominal surgery. Patients will receive supplemental parenteral nutrition or not within 7 days after abdominal surgery. Both groups will receive SPN except 80% of target energy delivered by EN or oral or patients are discharged from hospital. The primary and secondary outcomes will be collected.

Patients after abdominal surgery will attempt enteral nutrition support for 2 days, if she/he can not tolerate 30% of target energy, then she/he will be randomized to early PN group or late PN group at day 3. Patients in early PN group will receive supplemental parenteral nutrition at day 3 while patients in late PN group will not receive supplemental parenteral nutrition until day 8 after abdominal surgery. Both groups will receive SPN except 80% of target energy delivered by EN or oral or patients are discharged from hospital. The primary and secondary outcomes will be collected.

Inclusion Criteria:

1. Informed consent of patients or their legal representatives to participate in this study.
2. Patients undergoing selective operation without trauma
3. Patients following medium or major abdominal surgery
4. NRS 2002≥ 3

Exclusion Criteria:

1. Psychiatric disorders

2. Pregnancy or breast-feeding women

3. Malnutrition

   1. Weight loss >10%-15% in 6 months
   2. BMI<18.5
   3. SGA score with stage C
   4. Albumin <30g/L

4. Unstable vital signs or unstable hemodynamics (such as systolic blood pressure < 90 mmHg or mean arterial pressure < 70 mmHg after rapid 500 ml crystal or 200 ml gel infusion, or the 50% increase of vascular active drug infusion rate in an hour, etc)

5. Refuse to participate in the study

6. Mortality rates expected to more than 50% in 6 months with malignant or irreversibility diseases

   1. Cancer in terminal stage
   2. HIV positive at end-stage or CD4 < 50/mm3
   3. Cardiopulmonary resuscitation (CPR) before cardiac arrest and nervous system function not fully recovery
   4. Four levels of physical activity of the patients defined by New York heart association
   5. Rely on breathing machine because of chronic diseases

7. Life expectancy less than 24 hours of dying patients

8. Refractory shock to meet any of the following article

   1. The infusion rate of dopamine > 15 ug/kg/min
   2. The infusion rate of dobutamine > 15 ug/kg/min
   3. The infusion rate of epinephrine and norepinephrine > 30 ug/min
   4. The infusion rate of phenylephrine > 50 ug/min
   5. The infusion rate of milrinone > 0.5 ug/kg/min
   6. The infusion rate of vasopressin > 0.04 U/min
   7. Inter aortic ballon pump (IABP)

9. Hepatic insufficiency (alanine/aspartate transaminase 200% above normal range)

10. Renal insufficiency(creatinine 200% above normal range)

11. Metabolic diseases（hyperthyroidism/ hypothyroidism, adrenal cortex disorders)

12. EN can reach 30% of target energy in 48 hours after surgery

13. Burn area exceeding 20% of the patient's body surface

14. Autoimmune diseases or immune dysfunction or history of organ transplantation

15. International standardization ratio (INR) more than 3.0 or platelet count < 30000 cells/mm3 or other hemorrhagic diathesis

16. Intracranial hemorrhage one month before enrolment

17. General contraindications to infusion therapy or history of severe allergy against ingredients of enteral and parenteral nutrition

18. Has already participated in another clinical trial

19. Has started to nutritional support therapy before enrolment

20. Diabetes mellitus (anamnestic and/or under medical treatment)