Kentucky bluegrass (Poa pratensis) turf,which had been growing for 4 years at the south campus of Inner Mongolia Agricultural University,Hohhot,was mowed at four different heights (12,8,5 and 3 cm) with non-trimmed plants (relative height 18 cm) as the control.Five ecological parameters including carbon fixation,oxygen release,water release,heat absorption and cooling of lawn were measured between Aug. and Oct. 2019.Ecological benefits were comprehensively evaluated by the method of membership function analysis to investigate the optimum mowing height of leisure lawn in semi-arid areas.The results showed that the ecological efficiency of the five parameters decreased with the decrease in mowing height.According to the three-level system of comprehensive evaluation of ecological benefits,the unpruned control had the highest ecological benefit as Level I,followed by the trimming height at 12 cm and 8 cm as Level II,while the trimming height at 5cm and 3cm had the lowest ecological benefit as Level III.Considering the landscape effects,ecological benefits and the duration of landscape maintenance,our results suggested that the optimum mowing height of urban leisure lawn should be 8~12 cm.