Abstract: In order to analyze the effects of isotonic salt stress and water stress on the photosynthetic characteristics of tomato leaves, four kinds of genotype tomatoes with different salt tolerance (small fruit type croton agate, wild currant tomato, large fruit type Jintian powder crown, and supernatant 402) were selected to conduct water and isotonic tolerance experiments under 140 mmol·L^-1 NaCl and 15% PEG6000 simulating condition. The results showed that, 12 days after treatment, significant decreases in chlorophyll content (chlorophyll a, chlorophyll b, and total chlorophyll), photosynthetic rate, stomatal conductance, intercellular CO₂ concentration, and transpiration efficiency were observed, and the value of stomatal limitation was increased. Stomatal factors might reduce the decline in photosynthesis under the two kinds of stresses. The damage of the photosynthetic system under isotonic salt stress was greater than that under water stress, because water stress resulted in osmotic stress and ion damage. Among the genotype tomatoes, the salt-tolerant Lantau red agate exhibited high photosynthetic characteristics, and Jindian powder crown had the lowest photosynthetic efficiency. For the ultrastructure of chloroplasts, both stress treatments increased the stomatal density of tomato leaves, but decreased the numbers of open stomata. The length of chloroplasts was increased, the width was decreased, and the length/width ratios of chloroplasts were increased. In addition, number of chloroplast grana was decreased and that of osmiophilic globules was increased. The effect of salt stress on the chloroplast structure was greater than that of water treatment. The change in stomata of salt-tolerant currant tomato was less than that of salt-sensitive cultivar.