Abstract: To clarify the mechanism of sodium molybdate on promoting the carbon and nitrogen metabolism of tobacco seedlings and its effective application concentration, a pot experiment was carried out by culturing burley tobacco TN90 in an artificial climate chambers with temperature from 22 to 28℃, average photosynthetic photon flux density of 800 μmol·m^-2·s^-1, relative humidity of 85%, day (light) and night (dark)alternate every 12 hours. Sodium molybdate solutions at five concentrations (0, 0.1%, 0.5%, 1.0% and 2.0%) were sprayed on tobacco plants to study the treatment effects on the pigment content, photosynthetic rate, nitrate reductase activity, carbon and nitrogen compound contents of burley tobacco leaves. The results showed that within the concentration of 0.1%-2.0%, the pigment content, activities of enzymes correlated with nitrogen metabolism, soluble protein content, carbohydrate content and dry matter accumulation increased and the contents of total nitrogen and nitrate decreased in the leaves after spraying sodium molybdate. At the sodium molybdate concentration of 1.0%, the dry matter accumulation and nitrogen use efficiency of the leaves raised by 30.72% and 12.42% respectively, the nitrate content decreased by 27.45%. In conclusion, within the concentration of 0.1%-2.0%, the application of sodium molybdate promotes the carbon and nitrogen metabolism and nitrogen use efficiency of burley tobacco seedlings, significantly decreases their nitrate content, especially at the concentration of 1.0%.