Humic acid could improve the effect of fertilizers. Tomato was used as material in this experiment, the effects of humic acid synergist on the growth of tomato seedlings and the activity of nitrogen metabolism enzymes were studied in order to provide a reference for the development and application of humic acid. In this sand culture experiment, the humic acid was added to Hoagland nutrient solution with 0 (HA0), 1 (HA1), 2 (HA2), 5 (HA3) and 10 mL/L (HA4) respectively with 6 replications for each treatment. After culture for 30 days, the plant growth index, nutrients contents, nitrate reductase activity, glutamine synthase and glutamate dehydrogenase activities were measured. Adding proper amount of humic acid could promote the growth of tomato seedling, and the dry weight of root system of HA3 treatment increased by 31.68% than that of HA0. The dry weight of aboveground of tomato treated with HA3 was the largest. Adding Humic Acid could increase chlorophyll contents in Tomato leaves at the seedling stage. The contents of chlorophyll and carotenoid in tomato leaves treated with HA3 were the highest, which were 17.11% and 24.04% higher than that of HA0, respectively. The addition of proper amount of humic acid could increase the assimilation of nitrogen in roots and shoots of tomato at seedling stage. The accumulation of nitrogen in root, shoot and total nitrogen of tomato treated with HA3 was increased by 20.24% and 21.54% than that of HA0, respectively. The addition of humic acid could regulate the nitrogen metabolism process of tomato roots and leaves, and increase the activities of nitrogen metabolizing enzymes. Compared with HA0, the nitrate reductase activity of roots treated with HA4 was the highest, and the activity of glutamine synthase with HA3 was the highest. The maximum activity of glutamate dehydrogenase was found in root system treated with HA2. The activities of nitrate reductase, glutamine synthase and glutamate dehydrogenase in tomato leaves treated with HA3 were the highest, which increased by 38.27%, 64.54% and 106.63% respectively compared with HA0. The addition of humic acid could promote the growth and absorption of nitrogen and increase the activities of nitrogen metabolizing enzymes at tomato seedling stage. The best effect was found under the addition of 5 mL/L. When the addition was lower than 5 mL/L, it could promote the growth and nitrogen metabolism of tomato seedlings.