The effects of planting density (19 500, 18 000 and 16 500 plants/hm2) and nitrogen application level (105.0 and 127.5 kg/hm2) on microclimate of tobacco field and photosynthesis were studied. Results showed that leaf surface temperature was 2.50 ℃ higher than field ridge temperature on average, which were greatly influenced by planting density and there was a positive correlation between them (r=0.800). When nitrogen application amount was 127.5 kg/hm2, and with each increase of planting density by 1 500 plants/hm2, temperature at lower, middle and upper ridges decreased by 1.35, 1.65 and 1.95 ℃, respectively; and leaf surface temperature decreased by 1.80, 1.55 and 1.40 ℃, respectively. Planting density showed no significant effect on relative humidities between ridges, but shown negative significant interaction effect with the effect value of －0.538. Density affected the photosynthetic radiation in the middle and lower parts of tobacco plants, under high planting density, the increased application of nitrogen reduced the effective photosynthetic radiation in the middle and lower leaves by 39.46% and 56.98%, respectively. Density and nitrogen application significantly affected photosynthetic rate and intercellular CO2 concentration of tobacco plants with interaction effect, but did not affect leaf stomatal conductance and tobacco transpiration rate. Reduced density and increased nitrogen application raised photosynthetic rate of tobacco plants, the average photosynthetic rate of tobacco leaves planted at 16 500 plants/hm2 and with nitrogen 127.5 kg/hm2 was 23.09% higher than that in treatment with 19 500 plants/hm2 and nitrogen level of 105.0 kg/hm2. Nitrogen application rate was the main factor affecting intercellular CO2 concentration with positional differences. Regression analysis showed that stomatal conductance, intercellular CO2 concentration and transpiration rate significantly affected photosynthetic rate. Effective photosynthetic radiation was positively correlated with air temperature, leaf temperature and field relative humidity. Leaf temperature was positively correlated with leaf transpiration rate and field relative humidity was negatively correlated with leaf intercellular CO2 concentration. The results showed that planting density and nitrogen application changed the transpiration rate of tobacco plants by influencing the microclimate such as air temperature, leaf temperature and field relative humidity in tobacco fields, and then affected leaf intercellular CO2 concentration and photosynthetic rate ultimately.