Objective To observe the safety of high-dose vitamin C intravenous infusion in the treatment of advanced malignant tumors, and to provide reference for clinical rational use of vitamin C. Methods from April 2016 to June 2017, 21 patients with advanced malignant tumor were treated with high dose vitamin C in aviation general hospital, chinese medical sciences university. The symptoms of nausea, loss of appetite, abdominal distention, diarrhea, abdominal pain, dizziness, blood glucose, electrolytes (blood Na+, K+, Cl-) and liver and kidney function (alanine aminotransferase, aspartate aminotransferase, total bilirubin, urea, creatinine and Uric acid) were recorded in 21 patients with tumor before and after high dose vitamin C treatment. The paired test was then used to compare these patients' clinical symptoms and blood biochemical parameters after high-dose vitamin C treatment. Results there were no significant changes in blood glucose, electrolytes, liver and kidney function in patients with malignant tumor treated with high dose vitamin C intravenous drip. One patient complained of loss of appetite, one patient complained of nausea and abdominal distention, and one patient suffered from lower limb edema. However, no similar symptoms and manifestations were found when vitamin C was used again, and no diarrhea, abdominal pain, dizziness and other symptoms was observed. Conclusions High-dose vitamin C has not been found in the treatment of advanced malignant tumor patients with adverse reactions and the security is good.