To reveal the regulating mechanism of nitrogen on watermelon root under chlorine salt stress. a soil culture experiment was conducted to study the effects of different N levels (0, 0.1, 0.15, 0.2, 0.25 g/kg) on architecture, biomass, organic osmoregulation substances, activity, malondialdehyde of watermelon root under the chlorine salt stress. Meanwhile, principal component analysis was used for comprehensive evaluation of root growth situation. The results showed that root branch number, root activity, the total number of root volume could be used as the comprehensive index to evaluate influence of nitrogen on root growth under chloride salt stress, and alleviating effect of nitrogen level on chlorine salt stress was shown as N15 > N20 > N10 > N25. Compared with no nitrogen treatment, as nitrogen level was 0.15 g/kg, watermelon root biomass and the amount of dry matter accumulation were increased by 58.83% and 20.83%, proline and soluble sugar content were enhanced 98.33% and 70.37%, root activity was raised 29.44%, while malondialdehyde content was reduced by 40.30%, moreover, total root length, root surface area, root number, and branch number were increased 103.42%, 46.41%, 64.44%, and 87.80%, but total root volume and total root diameter was decreased by 23.05% and 40.15%, respectively. Curve equation fitting results showed that watermelon has ideal root system configuration and higher root activity when N rate is range in 0.15 –0.19 g/kg under chlorine salt stress. Visibly, it can conclude that a moderate amount of nitrogen helps watermelon to establish a good root system configuration, increase root penetration levels, and reduce cell osmotic potential and malondialdehyde content, maintain strong root activity, enhance biomass and dry matter, and thus alleviate high concentrations of chloride salt inhibition for the growth of watermelon.