Abstract: To study the sensitivity of tomato growth to water saving at different growth stages and different nitrogen application levels,through the greenhouse plot experiment,four irrigation levels and three nitrogen application levels were set. The response of the above-ground and underground growth indexes to different irrigation and nitrogen application levels was analyzed,and the effect of water saving and nitrogen reduction on water and nitrogen utilization rate of tomatoes was studied. The results show that the plant height,stem diameter and yield of tomatoes decreases with the decrease of irrigation and nitrogen application,all reach the maximum value under the treatment of W1 N1（ conventional water and nitrogen application）. However,the characteristic parameters of the whole root first increas and then decreas with the decrease of irrigation and nitrogen application,the total root length,fine root length,total surface area and total volume of tomatoes are the highest under W2 N2 treatment（ 50%water saving at the seedling stage and 300 kg/hm2 nitrogen application）,which are 33%,34%,46%and 67% higher than those of W1 N1,respectively. Conventional irrigation and nitrogen application will promote the development of root system at the surface layer（ 0-30 cm）,while water saving and nitrogen reduction will make root system deeper. The treatment of W2 N2 has the most developed root system,the highest utilization efficiency of irrigation water,and the higher nitrogen fertilizer partial productivity. It is the best combination of water saving and nitrogen reduction in this experiment that water（ 20%） and fertilizer（ 25%） resources are saved while slightly reducing production（ 8%）.