The activities of POX,SOD,PAL,and CAT were increased markedly after infected with TMV and CMV . Enzyme activities were different under different potassium contents. The enzyme activities were respectively decreased with infection. But the POX,SOD,POL,and CAT activities of the high potassium were higher than those of the control. These variations were the adaptive reaction to the infecting virus, and it might help the tobacco to eliminate large activated oxygen which was produced after infection so that improving the resistance to disease. The potassium could improve internal source protective enzyme activities and remain high enzyme activities for a long time. The phenomenon of decreased chlorophyll content and the increased MDA content and permeability of plasma membrane were controled by improved potassium status.Thus, it would improve the resistibility of tobacco leaf to diseases. The activities of POX, SOD, PAL, and CAT were significantly markedly after after infected with TMV and CMV. Enzyme activities were different under different potassium contents. These high potassium were higher than those of the control. These variations were the adaptive reaction to the infecting virus, and it might help the tobacco to eliminate large activated oxygen which was produced after infection so that improving the resistance to disease. The potassium could improve internal source protective enzyme activities and remain high enzyme activities for a long time. The phenomenon of decreased chlorophyll content and the increased MDA content and permeability of plasma membrane were controled by improved potassium status. It, it would improve the resistibility of tobacco leaf to diseases.