The invention discloses a control method for decreasing tobacco root and stem diseases. The control method includes: applying calcium oxide to stem basic parts of tobacco plants in a watering or dry manner during the later period of flourishing growth of the tobacco plants so as to achieve a good control effect on the tobacco root and stem diseases and improve the yield and the quality of tobacco leaves, wherein the amount of the calcium oxide is 225 kg/hm2. The control method can effectively prevent and control occurrence and damage degree of the tobacco root and stem diseases, can avoid chemical pesticides, and can provide a new thought for development of the harmless control technique and organic tobacco agriculture.