Tobacco brown spot is one of the most serious diseases in tobacco production in China leading to enormous losses in yield and quality. It is necessary to select resistant tobacco cultivars to meet the demands for high quality tobacco production, but there have only been occasional reports on the pathogenesis of tobacco brown spot in the past. It is therefore very important to study the disease mechanism of tobacco brown spot in order to establish recommendations and a foundation for breeding resistant cultivars. The responses of photosynthetic pigment content, photosynthetic parameters and chlorophyll fluorescence kinetic characteristics were investigated under three different stress levels of tobacco brown spot (mild, moderate and severe stress) at maturity using pot cultures. Two tobacco cultivars with different resistances to tobacco brown spot were selected as experimental materials to determine whether there was a correlation between photosynthetic characteristics and resistance. The cultivars selected were JYH (resistant cultivar) and CBH (sensitive cultivar). The results showed that tobacco brown spot stress could decrease chlorophyll a, chlorophyll b, total chlorophyll and carotenoids in the two cultivars with different resistances. In addition, the decreased range of photosynthetic pigments was less in JYH than in CBH. Tobacco brown spot stress decreased both the net photosynthetic rate and stomatal conductance. However, the net photosynthetic rate of JYH increased under mild stress whereas there was a decrease under moderate and severe stress. In the two cultivars with different resistances, the changes in intercellular CO₂ concentration and stomatal conductance were divergent. The intercellular CO₂ concentration in CBH tended to rise under the three different tobacco brown spot stress levels, whereas the stomatal limitation value showed a significant decrease. In JYH under severe stress, the photosynthetic parameters consisting of both intercellular CO₂ concentration and stomatal conductance showed the same trend as in CBH. However, compared with the rising trend of intercellular CO₂ concentration under severe stress, the stomatal limitation value of JYH decreased under both mild and moderate stress. With the increasing levels of tobacco brown spot stress, the initial fluorescence (F₀) and non-photochemical quenching (NPQ) of the two cultivars with different resistances both increased. Compared with controls (no stress), the increases in F₀ in JYH and CBH were 16.50% and 34.48%, respectively under severe stress, whereas NPQ increased by 95.54% and 137.45%, respectively, at the same time. For these two parameters, all the differences between severe stress and control conditions were significant for JYH and CBH. However, the chlorophyll fluorescence kinetic parameters including maximal fluorescence (F_m), variable fluorescence (F_v), maximal photochemical efficiency (F_v/F_m), PSⅡ potential efficiency (F_v/F₀), photochemical quenching coefficient (q_p) and PSⅡ actual quantum yield (Φ_PSⅡ) for the two cultivars with different resistances all decreased by varying degrees under the three levels of tobacco brown spot stress, and the decreased range was less in JYH than in CBH. These results indicate that tobacco brown spot disease had a destructive effect on photosynthetic characteristics. The photosynthetic pigments, photosynthesis and chlorophyll fluorescence characteristics of JYH were less influenced by tobacco brown spot stress than CBH. So a higher photosynthetic performance may be a major physiological trait of resistant cultivars in the adaptability to tobacco brown spot infection.