Low urine output after surgery may be caused by insufficient preoperative intake, excessive fluid loss during and after surgery, and anesthesia. 1. Insufficient preoperative intake: due to fasting and water fasting before surgery, or severe vomiting and diarrhea, etc., will lead to a decrease in body fluids, which will lead to low urine after surgery. 2. Excessive fluid loss during and after surgery: excessive blood loss during surgery, or excessive water loss due to hyperventilation after surgery without replacing the lost amount can also lead to postoperative oliguria. 3. Anesthesia: when the patient is under anesthesia, due to weak bladder contraction and no sense of holding urine, although the kidneys produce urine, it will be retained in the bladder, leading to the occurrence of postoperative oliguria. When postoperative oliguria occurs, you should contact the relevant doctor in time to clarify the cause and treat the symptoms under the doctor’s guidance.