Abstract: Aiming at the problems of water and NO3--N leakage that induced by irrigation practices in vegetable field, the relationship between furrow irrigation amount under the mulching and soil nitrate nitrogen amount that leaked out from the soil root layer was studied by combination of the field experiments and labor analysis in tomato field under greenhouse. The results show that the irrigation amounts are linearly related to NO3--N leakage amounts under both the NO3--N fertilization and no NO3--N application, and the irrigation practice could cause the NO3--N leaching definitely in 0-20cm root layer whether there is the fertilization. There is one peak value of soil nitrate nitrogen leakage amount in the range of 7.5mm-15mm water supply, while there are two peak values under the 22.5-45mm water supply. The NO3--N leakage rate is 0 under the no fertilizer application and 0-519% under the fertilizer application at the root layers the irrigation amount is from 75mm to15mm. Under the 22.5-45mm water supply, there is only 5.38%－19.08％ NO3--N leaked out of the root layers in the no fertilizer application, while the value is 21.91%-61.96% under the fertilizer application. The reasonable irrigation amount is 15mm for the greenhouse tomato in this research.