At Minqin oasis fringe, effects of descending water table to niche of plant population was studied using spatial difference of water table( 8-12 m in Huqu region, 15-17 m in Quanshan and 20-23m in Baqu) and the descending grades of water table during 1984 to 1992 (7.45-11.65 meter) in Sha-jin-zi. The result reveals: with the descending of underground water in different regions , the breadth of niche of plant population decreases, and the population degenerates; with the descending of water table in time grade , Nitraria tangutorum population extends, while the others decrease. Nitraria tangutorum, with the widest breadth of niche among the desert plant communities at Minqin oasis fringe, is the dominant species in this region. Nitraria tangutorum population generally extends at a water table ranging from 7.45 m to 11.65 m, so the critical water table for improving eco-environment of Minqin oasis should be within this scope.