Abstract: Two sweet potato varieties with different responses to sweet potato scab stress (“Guangshu 88-70”,a disease-resistant variety;“Jinshan 1255”,a disease-sensitive variety) were used as experimental materials to study the metabolism of photosynthesis of plants under sweet potato scab stress in field experiment.Under sweet potato scab stress,chlorophyll is decomposed;the contents of unsaturated membrane fatty acids,the index of the unsaturated fatty acids,and the membrane fluidity decrease.The activities of Mg²⁺-ATPase and Ca²⁺-ATPase of chloroplast decrease.But two sweet potato varieties own different decreasing rate of activities of ATPase.On the other hand,the stomatal conductance,net photosynthetic rate and transpiration rate decrease to different degrees. Intercellular CO₂ concentration of sensitive variety increases to a little degree and that of resistant variety decrease.The ultrastructure of chloroplast of disease-sensitive variety is much more decomposed than that of disease-resistant variety.