Excessivenitrogen(N)application is still very common in the greenhouse vegetablecultivation，where high -intensity inputs of water andfertilizers，and increasing planting area result in more and more serious environmental impacts and none-point source pollution.Increasing fertilizer efficiency and reducing the application rate has become a nationalmission.Urea ammoniumnitrate(UAN)is a new type of liquid N fertilizer which is applied in China recently with theincrease of fertigation facilities ingreenhouses，but how to using UAN efficiently in greenhouse systems is notclear.Here we conducted a field experiment with UAN and a mixed-type N inhibitor composed of ureaseinhibitor(NBPT)and nitrificationinhibitor(DCD)applied onlettuce，and five treatments wereCK(no Napplied)，UAN，UAN plus the mixed inhibitor at5 kg per ton ofUAN(ND5)，UAN plus the mixed inhibitor at 8 kg per ton ofUAN(ND8)，UAN plus the mixed inhibitorat 10 kg per ton ofUAN(ND10).Drip irrigation and fertigation were used，two times of fertigation with total N of 144 kg/hm²，four times of irrigation with total irrigation water of 140 mm for the whole growingstage.The results showed that adding the mixed-inhibitor to UAN had significant effects on theyield，quality oflettuce，andenvironment.The yields ofND5，ND8 and ND10 were 10%～21% higher versus UANtreatment，and the highest was from the ND8treatment；the leaf nitrate concentration and soil residual nitrate were also reduced with the mixed-inhibitor，which resulting in less nitrate leachingand lower environmentalload.Applying N fertilizer increased apparent Nlosses，however，there was a decline trend onthe apparent N losses in the ND8 and ND10 treatments compared with the UANtreatment.With the increment of the mixed -inhibitor，economic outputs rose firstly thenfell，and the output -input ratio became lower at the higher rate of the mixed-inhibitor.Based on thisstudy，the mixed -inhibitor at 8 kg/t UAN is the optimal dosage for improving lettuceyield，quality while reducing environmental impacts.