The responses of photosynthetic diurnal variation and net photosynthetic rates of tomato (processing type, cv. Riegel 87-5) during fruit enlarging stage to light density under three irrigation levels were studied to evaluate tomato photosynthetic capability and models suitable for tomato photosynthesis simulation in water deficit condition. The results showed that: (1) the gas exchange characters of tomato leaves under slightly water stressed had no significant difference with the control, but the net photosynthetic rate, stomatal conduction and evaporation rate decreased significantly under middle level water stress. (2) Four models of rectangular hyperbolic model, non-rectangular hyperbolic model, exponential model and modified rectangular hyperbolic model can all be used to simulate the apparent quantum yield, maximum net photosynthetic rate, dark respiration rate, light saturation and compensation points of tomato, but the modified rectangular hyperbolic model gave the best math fitting results compared with the measured values. (3) Under different water conditions, the apparent quantum yield (α), maximum net photosynthetic rate (P_nmax) were significantly positively related to the mean photosynthetic rate, and the mean photosynthetic rate was significantly and positively associated with fruit yield. The mean photosynthetic rate, and ultimately the fruit yield decreased under moderate water stress. While slightly deficit irrigation did not decrease fruit yield of processing tomato significantly and resulted in higher water use efficiency.