Professor LI Siwen, a famous doctor of Traditional Chinese medicine (TCM) in Yunnan province, specializes in the treatment of patients with tumor with diabetes mellitus. This paper introduces Professor Li Siwen's successful experience in the treatment of lung cancer, gastric cancer and non-Hodgkin lymphoma with diabetes mellitus. The prescription reflects Professor LI's treatment characteristics: caring for spleen and stomach, benefiting qi and nourishing yin, activating blood circulation to dissipate blood stasis. It can be seen from Professor LI's experience in the treatment of tumor with diabetes mellitus that it is very important to effectively control blood sugar and reduce complications for the treatment of tumor with diabetes mellitus.